I often see the term "prior art" in relation to patents. What does it mean?

To patent something, it should be an original technique, or an original extension to an existing technique. The catalogue of existing known techniques is known as the prior art. A well written patent contains a list of references to relevant prior art. Cataloguing this helps make the patent more solid. If it is catalogued in the patent, it should have been taken into consideration at the time the patent application was examined, and the patent issued. It is then very difficult to use that prior art to pick holes in the validity of the patent. If the patent application was submitted in good faith, the submitter has no reason to avoid listing every relevant item of prior art they know about. After all, their techniques must go beyond the prior art to be valid in a fresh patent claim. However, many patents do not properly catalogue prior art. Many only catalogue prior art from other patents, ignoring techniques openly published in a non-patented form. In theory, you cannot patent something someone else has published. In most countries you cannot even patent something you have published. In some, like the USA, you can apply for a patent for a limited period after you first publish. In this context, publish might just mean you have used the technique in a commercially shipping product.