This term, along with the term pure-bred dog stems from a fundamental misunderstanding of the genetics and misinterpretation of Darwinian theory, and should be viewed with the same scepticism as we view the concept of pure races in human populations. The term cross-breed refers to puppies produced by the breeding technique of breeding known dogs of two different pure-bred dog breeds for some specific reason.
The term mutt generally refers to a dog of unknown descent. Dogs interbreed freely, except where extreme variations in size exist, so mixed-breed dogs vary in size, shape, and colour, making them hard to classify physically.