key word

noun
  1. 1

    Any word used as the key to a code.

  2. 2

    Any word used in a reference work to link to other words or other information.

  3. 3

    A reserved word used to identify a specific command, function etc.

  4. 4

    Any word that occurs in a text more often than normal.

Source: https://en.wiktionary.org/wiki/key%20word, https://en.wiktionary.org/wiki/keyword