You could probably find the veterinary definition in a text book or on a
vet advise web site. That said.... It's an area of skin break-down
caused by any of several insults to the skin. Flea bite allergy, bee
sting, skin trauma, constant dampness (not drying out between swims) are
among the culprits, usually combined with some chewing or***ing by the
dog.... or by scratching if out of range of the mouth.