Query expansion (QE) is a process in Information Retrieval which consists of selecting and adding terms to the user’s query with the goal of minimizing query-document mismatch and thereby improving retrieval performance.
This is generally useful when a search phrase is too short, which often means that the user is relying on implied knowledge that the full-text search engine lacks. For example, a user searching for database may really mean that MySQL, Oracle, and RDBMS all are phrases that should match databases and should be returned, too. This is implied knowledge.