I entered your question into Google and clicked on the top link.... I suggest you do the same, here are the first two paragerphs.
Searching for Adequacy & Reasonableness
A search for responsive discovery is reviewed on whether the proffering party has “demonstrated that it has conducted a search reasonably calculated to uncover all relevant documents.” Mullen v. United States Army Crim. Investigation Command, 2012 U.S. Dist. LEXIS 93977, 12-15 (E.D. Va. July 6, 2012), citing Rein v. United States PTO, 553 F.3d 353, 362-63 (4th Cir. Va. 2009).
The producing party can demonstrate an adequate search with the following:
…[An] affidavit [that is] reasonably detailed, setting forth the search terms and the type of search performed, and averring that all files likely to contain responsive materials (if such records exist) were searched so as to give the requesting party an opportunity to challenge the adequacy of the search.
Mullen, at *13, citing Rein at 362-63.