keyword

Concise Oxford English Dictionary © 2008 Oxford University Press:
keyword/ˈkiːwɜːd/

▶noun

• 1 a word which acts as the key to a cipher or code.

• 2 a word or concept of great significance.

  ■ a word used in an information retrieval system to indicate the content of a document.

  ■ a significant word mentioned in an index.