A word or phrase (although many keywords are actually short phrases, they are almost universally referred to as keywords) for which a person has searched, using a search engine such as Google or Bing, or any other type of search platform (such a site’s internal search system).
Keywords are the building blocks of any search engine optimisation or pay per click campaign. Selecting the correct keywords to target can make or break it.
“Key phrase”, “search query” (or just “query”) and “search term” mean the same thing.