Word usage is how a word, phrase, or concept is used in a language. Lexicographers gather samples of written or spoken instances where a word is used and analyze them to determine patterns of regional or social usage as well as meaning. A word, for example the English word "donny" (a round rock about the size of a man's head), may be only a rare regional usage, or a word may be used worldwide by all English speakers and have one or several evolving definitions.
Word usage may also involve grammar.
References
Word usage Wikipedia(Text) CC BY-SA