Here's the official definition from AskOxford.com:
keyword
• noun 1 a word which acts as the key to a cipher or code. 2 a word or concept of great significance. 3 a significant word mentioned in an index. 4 a word used in an information retrieval system to indicate the content of a document.
source:http://www.askoxford.com/concise_oed/keyword?view=uk
