keyword

Short definition from Wiktionary

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
   Any important word in a text or document, which may be linked to other words or other information, or listed in the metadata for searches to find.
4. 4
   A reserved word used to identify a specific command, function, etc.
5. 5
   Any word that occurs in a text more often than normal.

Verb

1. 1
   To tag with keywords, as for example to facilitate searching.