Answer:
prior art
Explanation:
Prior art refers to certain knowledge that was obvious with or without a patent. In other words, it is like someone trying to patent the wheel, since people know about, know how it works, but no one has gotten a patent for it. If you go a patent office and try to look for the patent of a wheel, you will not find it, but that doesn't mean that if you patent it, you will successful defend your patent.
