“Invalidity Search” and “Validity Search” are essentially the same. Both these searches are targeted to find relevant prior-art documents which may be used to invalidate the claims of the target patent. Specifically, the invalidity search is used to find prior-art documents to invalidate the claims of the target patent; and the validity search is used to establish enforceability of the claims of the target patent.
Invalidity search may be performed by a business, say, in case of a lawsuit filed against it for infringing a patent. Alternatively, a business may want to “validate” its own patent by confirming that others could not find invalidating prior art, say, before initiating an infringement suit against a third party.
It may not be important whether you call it “invalidity” or “validity” search, this is typically a matter of perspective; although it is best to use the same term as used by the client. The same search process is used in both cases, but the client’s desired outcome is different. The client particularly wants you to find references in the first case, but may be secretly wishing that you do not come across any relevant prior-art in the second case.
Ideally, no amount of prior art can “prove” that the patent is valid and should have been granted. There is always a possibility of some prior-art document lurking in the corner which may not have been found in your search, no matter how exhaustive. On the other hand, an exhaustive prior-art search can be effective in allowing patent attorneys to make the case that a patent is invalid and therefore unenforceable. That may be why that most attorneys prefer to use the term “invalidity search” for such prior-art searches.
A searcher is expected to take use all possible resources, including sifting through hoards of non-patent literature, to find critical prior art which may have been overlooked by the patent examiner during prosecuting and grant of the patent. Such prior-art would allow the client to challenge another’s patent (“invalidity”) or make them prepared to defend its own patent against by a competitor (“validity”).