A term used by, in particular, the USPTO to refer to fields in which there are a lot of patents. In particular, the MPEP §904.02 states:
“In crowded, highly developed arts where most claimed inventions are directed to improvements, patent documents, including patent application publications, may serve as the primary reference source. Search tool selection in such arts may focus heavily on those providing patent document coverage.”
Unfortunately, it would appear that in many fields, particularly software patenting, US patent examiners frequently treat all inventions as being in a “crowded art” and thus frequently fail to look for non-patent prior art.