Perceptual encryption is a term used to describe various applications of encryption methods intended for audio, speech, image and video data.
The basic idea is that one performs encryption for multimedia content in a way that only a certain amount of "perceptual information" is touched by the encryption. It may be considered as intentionally degrading the quality of the multimedia content by means of encryption, whereas the part of the data that remains untouched can be processed by the respective decoder without requiring access to the encryption key. However, the full "perceptual information", i.e., the full quality multimedia data, can only be recovered when given access to the the encryption key.
For instance, one may use perceptual encryption to downgrade the quality for a free online preview of some movie and only if one pays for the content one obtains access to the encryption key and thus to the full quality content.
There are various freely available
articles describing different techniques for this type of encryption (just do a Google or Google Scholar search) and also some parts of books (you can find via Google Books), e.g.:
and just search for "perceptual encryption". I am not sure if there is something like a single definition because there are various techniques for perceptual encryption, but you should find classifications in the linked books.