Word index is a term used to refer to a list of words and their location within a document or publication. There are several synonyms for this phrase, including a keyword index, a vocabulary index, a glossary, and a lexicon. A keyword index is a list of the most important words or phrases within a document, while a glossary is a list of specialized terms and definitions. A vocabulary index is similar to a word index, but it may also include synonyms and related words. A lexicon is a comprehensive reference tool that includes all of the words and terms used within a language or field of study.