Taut, hard, solid, versus slack, soft, amorphous—language.
On the one side is Strunk & White’s Omit needless words which omits needless words in itself (and therefore is a an autological phrase). On the other side would be a paraphrase of the same idea: When you can, cut words that do not contribute to your meaning.
Each density of style—to coin a name for this taut-slack property—may be obviously assessed on the page, but like a lot of stylistic properties it is hard to define objectively.
For me, density is the rate of surprise, word for word and idea for idea. The more easily I can predict what comes next, the looser the text. The more surprised I am by what comes next, the denser the text.
A dense style needn’t be terse or cryptic. E. B. White of the Omit needless words follows his own dictum assiduously, but does not shy away from sentences fifty words long. This is the beginning of Death of a Pig (found in Essays). Note that polysyndeton, the proliferation of and in the quote, may appear deceptively “loose”, but actually introduces a new idea four out of five times (those are in bold).
I spent several days and night in mid-September with an ailing pig and I feel driven to account for this stretch of time, more particularly since the pig died at last, and I lived, and things might easily have gone the other way round and none left to do the accounting.
On the other hand, a dense style can be terse, cryptic, and punctuation heavy. Here’s Roland Barthes speaking about The Pleasure of the Text. (Translated from the French by Richard Miller.)
The text you write must prove to me that it desires me. This proof exists: it is writing. Writing is: the science of the various blisses of language, its Kama Sutra (this science has but one treatise: writing itself).
Density isn’t just a property of non-fiction.