Query expansion (QE) is the process of reformulating a given query to improve retrieval performance in information retrieval operations, particularly in the context of query understanding. In the context of search engines, query expansion involves evaluating a user's input (what words were typed into the search query area, and sometimes other types of data) and expanding the search query to match additional documents. Query expansion involves techniques such as:

• Finding synonyms of words, and searching for the synonyms as well
• Finding semantically related words (e.g. antonyms, meronyms, hyponyms, hypernyms)
• Finding all the various morphological forms of words by stemming each word in the search query
• Fixing spelling errors and automatically searching for the corrected form or suggesting it in the results
• Re-weighting the terms in the original query

Query expansion is a methodology studied in the field of computer science, particularly within the realm of natural language processing and information retrieval.