A mutt is any dog that's a combination of different breeds, rather than one identifiable breed. Your beloved mutt might be as big as a Golden Retriever but with short, Corgi-like legs.
Most dogs are mutts, and many of their owners are fine with the word mutt, despite the word's original implication of "inferior, mixed breed dog." Dogs called crossbreeds are deliberately bred to combine characteristics of two or more recognized breeds, while mutts — also known as mongrels — are the result of accidental mixing of breeds. In the late 1800s, mutt was a derogatory term meaning either "stupid person" or "stupid dog."