Matter that has been exposed to the public domain and as a result is not patentable.
Prior art does not require a physical existence, or have to be commercially available. The mere publication of any previous description of an invention is enough to satisfy its requirements. Many ideas or inventions are never commercialised, and yet there will be evidence of them somewhere in the public domain. This is enough to satisfy the notion of prior art. The implication of this is that if prior art exists, the inventive step required in a patent application will not be satisfied, and as a result a patent will not be granted. The potential existence of prior art highlights the importance of the nominal search phase of the patent application process. Extensive research of existing inventions that may contain similarity to your invention is therefore required. This will reduce the risk for potential patent holders of wasting time and money on a patent application that cannot be granted due to the existence of prior art.