Prior art is any evidence that your invention is already known.
Prior art does not need to exist physically or be commercially available. It is enough that someone, somewhere, sometime previously has described or shown or made something that contains a use of technology that is very similar to your invention.
A prehistoric cave painting can be prior art. A piece of technology that is centuries old can be prior art. A previously described idea that cannot possibly work can be prior art. Anything can be prior art.
An existing product is the most obvious form of prior art. This can lead many inventors to make a common mistake: just because they cannot find a product containing their invention for sale in any shops, they assume that their invention must be novel.
The reality is very different. Many inventions never become products, yet there may be evidence of them somewhere. That evidence - whatever form it may take - will be prior art.
There are no accurate statistics, but some experts estimate that for every recorded invention that eventually reaches the market, ten never will. This means that if you want to find out if your invention is novel, you should indeed search products past and present - but you should also search much further.
The most important place for further prior art searching is the worldwide patent system. Some patent databases - including the European Patent Office's free database Espacenet - contain 90 million documents, collected and indexed over many years by patent offices in many countries.
Thanks to the internet, and to the international classification systems used to organise inventions by subject, it is quite easy for inventors to do their own patent searching. We describe how to do this later.
While looking for prior art, you should also look for competing art. These are ideas that may not be at all like yours but do the same job. It is important to study competing art for two reasons: