What is Prose Density?

Prose density is the amount of work each word and sentence does. Dense prose carries more meaning per unit of language than its length seems to allow. You read a short sentence and find it holds an argument, an image, a judgment, and a turn, all at once. Loose prose does the opposite. It uses many words to deliver little, and you could cut half of it without losing anything.
Two roads lead to it, and they look like opposites.
The first is addition. A writer packs the sentence with loaded nouns and verbs, with allusion, with subordinate clauses that hold several ideas in tension. Samuel Johnson and Edward Gibbon write this way. A single Gibbon sentence can balance a fact against an irony against a moral verdict, and the architecture of the clauses does the thinking. The density comes from how much the writer fits inside the frame and from how the parts press on each other.
The second road is subtraction. Hemingway built density by leaving things out. He called it the iceberg. The strength of a passage comes from what stays under the surface, the seven-eighths the reader supplies. A plain sentence reads as dense when the writer has cut every slack word and trusted the reader to infer the weight. “He did not love her anymore” carries more than three pages of explanation, because the flatness forces the reader down into it. Omission compresses. So does the loaded plain word, the concrete verb that drags its connotations behind it.
Either way, the test is the same. Try to cut a word or unpack a phrase. If the meaning thins, the prose was dense. If nothing is lost, it was padded.
Allusion is a fast way to raise density, since one phrase opens onto a whole tradition. When a writer calls a man Augustinian, the single word carries a theology and a temperament and saves a paragraph. Rhythm raises it too. A sentence that lands on a stressed beat or closes on a reversal makes the sense and the sound arrive together, and the reader feels the point before he has finished parsing it. The aphorism is density at its limit, a full argument folded into a line that turns at the end.
The cost is real, which is why density is a setting, not a virtue you crank to maximum. Prose that stays dense for a whole page goes airless. The reader cannot breathe, cannot rest, cannot tell the large point from the small one because everything arrives at the same pressure. Good writing varies it. A dense sentence earns its weight when a plain one precedes it and a plain one follows. The contrast is what lets the reader feel the load. That is the deeper reason for mixing long sentences with short ones. The short sentence clears the ground. The long one builds. The density lives in the difference between them.