1. A significant word used in indexing or cataloging.
2. A word that is used as a pattern to decode an encrypted message.

A main word used in indexing.
1. A characteristic word, phrase, or code that is stored in a key field and is used to conduct sorting or searching operations on records in a database. See also key field.
2. Any of the set of words that composes a given programming language or set of operating-system routines. See also reserved word.