1. A computer-implemented method for searching using an adaptive thesaurus, comprising: receiving a search query including a query term; retrieving, from the adaptive thesaurus, an expansion term stored in association with an index term matching the query term, wherein metadata is stored in association with the expansion term and the index term, and the metadata includes: a recall gain meeting a recall gain threshold, wherein the recall gain identifies a value of expanding the search query to another term versus a computational cost; an expansion independence meeting an expansion independence threshold; and a semantic similarity meeting a semantic similarity threshold; and expanding, using a processor, the search query using the expansion term.