Pick up that little section of lead pipe; feel it in your hand. It’s heavier than you might’ve thought, maybe? It’s dense, in other words, right? Right.
Now, pick up that chunk of pumice, about the same size. What do you call that, in comparison to the pipe? Light? True, it is lighter than the pipe, but it could be just as heavy, if only it were bigger. Airy? Maybe, but even in a vacuum it would feel like it had the same weight, most likely. Sparse? No: It’s not a forest in your hand, after all.
The question is, when we’re talking about material objects, what’s the opposite of dense?
The answer, at the moment, is less dense, unsatisfactory because of our habit of using paired descriptions for physical properties (heavy/light, soft/hard, wet/dry, and so on). Once upon a time, though, dense had an opposite. Until the mid-19th century, the word was rare.
In fact, the earliest meaning of rare in English was precisely this one. The OED notes the word’s meaning as “[o]pposed to dense,” and provides the following example (from around 1420), among others:
The londis fatte, or lene, or thicke, or rare.
I don’t know what a londis is, and I’m intrigued by thicke as a stand-in for dense (as in a thicket, perhaps). But mostly, I’m thrilled to find rare used in this way.
One can imagine how, in the less dense Britain of the 15th century, a word like rare could move from describing a single object (as in, for example, “this rare piece of pumice”) to describing a collective object (“this rare forest”) to describing the likelihood of encountering individual objects while wandering through the realm (“the rare African swallow”).
Still, I propose we resuscitate rare‘s original meaning. I’ve looked for such a word many times, and so have others. To inspire you, I close with more examples from the OED:
Cvcumber in this mone is sowen rare. [C. 1420]
The Assemblie was so rare that they were not exceeding the number of nineteen Commissioners. [1610]
A projectile would travel a far greater distance through a rare medium like air, than through a dense medium like water. [1862]
Who wouldn’t want a word that can as readily describe cvcumbers as Commissioners, projectiles as londises? The chance to improve our language by looking to antiquated usage is (forgive me) a rare one.
Don't Look Now 
The term “dense” itself is somewhat of a misnomer, as we discussed yesterday. Who knows how a measurement became an adjective. Something could be more dense or less dense, just as something has length, temperature, weight, et al. where long and short, hot and cold, heavy and light are the appropriate poles. Good luck reviving “rare.” May I suggest “dord” to replace “dense,” thereby unencumbering it from “density”?
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How about sparce??
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Hi Gary — As I mention in the post, sparse only works for certain cases, where the question is about the density of a collection of objects in a given area or volume. Sparse doesn’t really work for, say, a single object that seems light given its size.
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When you consider the density of the single object is due to how tightly packed the atoms are that compose it, sparse works just fine.
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Except we discuss those obects on a macroscopic scale. I don’t have a glass of many water (molecules), I have a glass of much water.
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Interesting thought. To take your weight example, something can have more or less weight, but it can also be heavier or lighter.
Could it be that there was once a word comparable to weight that described the dense/rare spectrum with no special affinity for one pole or the other?
Dord is incredible. And the guy’s right: Why not?
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I think that the word “rare” is still in use. What I have encountered more frequently is “rarefied”, often used in the sense of a gas made less dense.
My son asked me the opposite of “dense” just today, and I am happy to have passed on this word to the next generation. 🙂
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That’s excellent news Refael! Glad to hear a new generation is curious.
And I’d forgotten about rarefied—nice work.
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I know this is late, but google just brought me here today.
Why not bouyant?
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Glad you found me here, Aaron, and don’t worry about being late. English moves slowly enough that the issue’s still as relevant as it ever was!
As for your question, I’d argue against buoyant because it refers specifically to the property of floating. It means “less dense than a particular medium in which the object being described is floating or would float”—typically water, but sometimes air (as in the case of helium balloons).
It’s perhaps akin to using serious as the opposite of light: It’s the right word sometimes, but not all the time. Rare, I think, covers many more cases—including buoyancy (which it can imply, which a little extra work), for example, and sparseness (as in the “rare forest” example).
Of course, if you’re using dense to describe a person who’s slightly slow to catch on to things, rare won’t work at all. It seems appropriate mostly for physical properties (but I’ve not heard buoyant used outside the realm of the physical, either).
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I appreciate your thoughtful responses. I can see ‘rare’ causing some confusion in statements such as
Helium is used to fill balloons because it is such a rare gas.
The Earth’s mantle is made of dense rocks such as olivine, but on the Earth’s surface these rocks are rare.
A dense fog is quite rare.
Either way, it is good to know that I am not the only person bothered by the lack of the perfect word. Concentration, which is very similar physically, has dilute, saturated, supersaturated, and concentrated to describe various points along its spectrum.
For another person’s attempt at a word to fill this linguistic vacuum, here is some context on ‘londis’.
Latin
Sed cum sint genera terrarum plurima, ut pinguis aut macra, spissa uel rara, sicca uel umida et ex his pleraque uitiosa, tamen propter seminum differentiam saepe necessaria maxime, sicut supra dixi, eligendus est pinguis ac resolutus ager, qui minimum laborem petit, fructum maximum reddit…..
Middle English
The londis fatte, or lene, or thicke, or rare, Or drie or moyst, and not withouten vice—ffor dyuers seed, yet, they right needful are…..
English (as translated in 1807)
But as there are many sorts of earth, as the rich or poor, dense or loose, dry or moist, and most of these inelegible……
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Your points about the ambiguity across these two meanings of rare seem right on the money to me. And yes! English as a language is so muddled that there is almost never a “perfect word,” as you put it. It’s frustrating and at the same time, leads to rich conversations like this one. And as a creative writer, I have to say I’m glad for it, because it allows me some play in the language I choose that might not be there otherwise. (I’m not quite good enough with any other languages for this to be a fair comparison, but all the same.)
Thanks for your comments, and also for the interesting backstory on the word. It’s been a long time since I took Latin, but it was nice to spot the rara in there among the rest.
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“Meat is very calorie dense, while fruits and vegetables are generally calorie rare.” The fact that I couldn’t phrase this as I wanted was killing me – until I came across this archaic little gem. I’m going to start using rare in this context whenever I can, and if anyone questions it I’ll just point them to this site.
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Excellent! So glad I could help, Will.
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Great post! Wonder why it left usage? Often the lack of a word says much about a language.
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Thanks Leiah! Glad you liked the post. And that’s a great question, why the word fell out of favor.
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I’m way late to the game, but a google search brought me here. I have come across this problem innumerable times (physics major) and have always used context to find a synonym for “not dense”, usually diffuse. I like rare, though. Its especially difficult to talk about things like aluminum as “diffuse” as compared to lead; I’ll use rare from now on.
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Hi Christian — That’s great! Glad to hear my research has some practical applications. Thanks for letting me know.
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Hi Devan,
I came across your discussion on the want of a word for not dense. Several people suggested words which might have previously had that meaning. I was researching the word subtile (subtle) tonight and found this in the Online Etymology Dictionary.
subtile (adj.)
late 14c., “clever, dexterous, crafty; not dense, thin, rarefied,” from Old French subtil (14c.), a learned Latinized reformation of earlier sotil (12c.), source of subtle (q.v.). Still used in some Bible translations in Gen. iii:1, and it survived after 17c. as a parallel formation to subtle in some material senses (“fine, delicate, thin”).
The reason I was researching subtle was the use of the name “Le Subtile” in reference to the character Magwa in the movie The Last of The Mohicans
I too have often lamented the want of such a word.
BTW the Online Etymology Dictionary is a great resource for those interested in the origins of words.
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Thanks John! Great to have more options…
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I know it is old and all, but I think it is one of the weakness of the english.
In portuguese when we want to say that something is not dense we say that it is “volumoso”. something that has “volume” occupies more practical space without being heavier than things that occupies less space. Volume is a cognate word, meaning it is the same written word with the same meaning in portuguese and english languages.
In other words the opposite of dense could be some gramathical interpretation of “something that has more volume”
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Very interesting Victor! I’d love a word like “volumous” in English.
We have “voluminous,” but it means something different, conveying a large volume independent of density. (You can even find contemporary descriptions of things as both voluminous and dense, as in a 1998 publication from the Geological Society of London.)
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I’d like a single-syllable word for such a basic and potentially visceral physical property. “Doss” sounds good to me as an opposite of “dense”, but it already has a pretty good meaning that our sleep deprived society could make better use of (sleep in rough accommodations; though I think I’ll use it to describe shitty sleep in general).
“Volumous” seems like it will work intuitively in this context. It might work best with a short u, so that the connotations between “volume”/”voluminous” and “massive”/”lage”/”loud” are toned down.
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Thanks for the note! I’m fond of “rare” (the historical opposite of “dense” in the material sense” for just that reason — one syllable sounds great. (I propose “rare” in the article but looking back, it’s easy to miss. Just made some key sentences bold.)
“Volumous” to me is a great idea but in practice I do worry people would read it as a typo, where “rare” at least might prompt them to look the word up. But then, I work with a lot of editors now, so maybe I’m gun-shy on the neologisms…
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I forgot you proposed “rare” after reading through all of the comments (and realizing I cared about the topic enough to post a reply). I apologize for imposing my omission.
I feel like I’d be overly conscious about avoiding the word’s common meaning, but maybe I’m underestimating how well it could work.
When it comes to gunshy neologisms I’m not really qualified to have a well-founded position on the matter, but I’ve never been able to shake the feeling that there might be tremendous consequences to the general avoidance. Too many people neglect the practices of song, dance, sports, etc. because their performances won’t be marketable. There’s so much between a box of crayons and a masterpiece painting that’s too often subjected to formal scrutiny and informal ridicule, sometimes in suspiciously equal amounts. What if there are entire tones of relatively basic concepts that don’t function well in mundane communication because we have the persistent need for some authoritative and/or hyped-up premise to be behind any new useful word before we even experiment with it confidently?
Sorry if that paragraph was both too dense and too rare at the same time. I’m going to go doss until the 3AM garbage truck wakes me up even through the densest earplugs I could find. (Speaking of which, have you considered words related to “foam”?)
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I know I’m late to the vibrant Anti-Density Symposium here (first comment in 5 years!) but even in late 2021, I’m on the same quest looking for the opposite of density. I don’t have anything positive to contribute, except removing the negative of the word “dord”
The Online Etymology Dictionary itself debunks the word as a clerical error.
https://www.etymonline.com/word/dord#etymonline_v_25793
dord (n.)
a ghost word printed in the 1934 “Webster’s New International Dictionary” and defined as a noun used by physicists and chemists, meaning “density.” In sorting out and separating abbreviations from words in preparing the dictionary’s second edition, a card marked “D or d” meaning “density” somehow migrated from the “abbreviations” stack to the “words” stack. The “D or d” entry ended up being typeset as a word, dord, and defined as a synonym for density. The mistake was discovered in 1939.
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Amazing story — thanks for sharing!
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