A defining vocabulary is a list of words used by lexicographers to write dictionary definitions. The underlying principle goes back to Samuel Johnson's notion that words should be defined using 'terms less abstruse than that which is to be explained', and a defining vocabulary provides the lexicographer with a restricted list of high-frequency words which can be used for producing simple definitions of any word in the dictionary.

Defining vocabularies are especially common in English monolingual learner's dictionaries. The first such dictionary to use a defining vocabulary was the New Method English Dictionary by Michael West and James Endicott (published in 1935), a small dictionary written using a defining vocabulary of just 1,490 words.