Synonym Expansion
Synonym expansion automatically matches a query to products using known equivalent terms — like "sneakers" and "trainers" — even when the shopper's exact word doesn't appear in the product data.
Part of Search Recovery
It addresses the vocabulary mismatch problem directly: a catalog tagged consistently with one term will still return zero results for a shopper who searches a synonym, unless the search system maintains a synonym list (or infers equivalence semantically) and expands the query before matching.