Ringworm \Ring"worm"\, n. (Med.) A contagious affection of the skin due to the presence of a vegetable parasite, and forming ring-shaped discolored patches covered with vesicles or powdery scales. It occurs either on the body, the face, or the scalp. Different varieties are distinguished as Tinea circinata, Tinea tonsurans, etc., but all are caused by the same parasite (a species of Trichophyton).

Source: Webster's Revised Unabridged Dictionary (1913)