Validity searches (invalidity searches, invalidation searches, opposition searches, prior art searches) are aimed at finding publications or other disclosures based on which it may be argued that, at the time of filing of a patent application, an invention was already known or at least obvious to a person skilled in the art.
As such, validity search reports can be used to decide what the best course of action is when confronted with third party patents or pending patent applications that are relevant to the technical field of the business you're in.
Other terminology being used for the same type of search: invalidity search, invalidation search, opposition search, prior art search.
NPS Patent Searches
Quaerite et Invenietis