A loss of mucous substance of the mouth showing local excavation of the surface, resulting from the sloughing of inflammatory necrotic tissue. It is the result of a variety of causes, e.g., denture irritation, aphthous stomatitis (STOMATITIS, APHTHOUS); NOMA; necrotizing gingivitis (GINGIVITIS, NECROTIZING, ULCERATIVE); TOOTHBRUSHING; and various irritants. (From Jablonski, Dictionary of Dentistry, 1992, p842)