Lexical density is term used in text analysis. It measures the ratio of content words to grammatical words. Content words are nouns, adjectives, most verbs, and most adverbs. Grammatical (sometimes called functional) words are pronouns, prepositions, conjunctions, auxiliary verbs, some adverbs, determiners, and interjections.
The content words are most important for explaining information. If you have a high number of content words, you’ve probably written a specialized academic text which will only be understood by well-educated people in that specific field. If you have a low number of content words, you have a very simple, easy-to-understand piece. If the number of content words is too low, then your writing may not adequately explain the premise of your text.
Lexical density also considers the number of unique words. If you’ve re-used words, you’ve reduced your lexical density.
These sentences are lexically dense:
The quick brown fox jumps over the lazy dog.
What a piece of work is man.
It’s better to give than to receive.
These sentences are not lexically dense:
If I were you I wouldn’t do that with these.
No, not me.
The thing… that thing…?