noun
: either of two forms of a virus disease of chickens, turkeys, and various other birds characterized by head lesions:
a. : a cutaneous form marked by pustules, warty growths, and scabs especially on the unfeathered skin — called also sorehead
b. : a more serious form occurring as cheesy lesions of the mucous membranes of the mouth, throat, and eyes that sometimes coalesce into a false membrane — called also avian diphtheria