(From Dog General - I'm sure that this can apply to cats also):

I am just doing some research into the definition of a Mixed breed pet and a Cross breed pet. Would you describe a CROSS breed as a pet that comes from two parents of known and/or identified BREED, therefore it is a CROSS of two identifiable parents? If so, would you then describe a MIXED breed as a MIX of unknown and/or unidentifiable heritage? Or would you say that it should be the opposite way around - a cross is unknown heritage and a mix is known heritage? I have posted this in a different MB and have had interesting answers, but just wanted to see what the general population here think.

Now I'm going to throw another spanner in the works here and add a term that I have thought up: a BLEND breed. How I'm thinking the best way to describe it is as follows:
Cross breed: cross of two purebreds.
Mixed breed: mix of parents of unknown heritage.
Blend breed: blend of more than two breeds of known parentage.

Thanks in advance, I look forward to some interesting opinions