A defining vocabulary is a published, stable, and culturally accepted core glossary specifically used by dictionary publishers to standardize their use of simple words to explain complex words, and culture-specific idioms or metaphors. It can also be published as a defining dictionary, but the most common use of such dictionaries is to assist in creating new dictionaries.

An example of a useful published vocabulary is Basic English (850 words). The defining vocabulary used by Longmans to define its 4000 most common English language idioms is about 2000 words long. The English variant E-Prime is designed to avoid any judgemental statements, and so also may be useful for a neutral defining vocabulary.

See also: dictionary, core glossary, Basic English, E-Prime