Medical Definition of blackhead
1: a small plug of sebum blocking the duct of a sebaceous gland especially on the face—compare milium
2: a destructive disease of turkeys and related birds caused by a protozoan of the genus Histomonas (H. meleagridis) that invades the intestinal ceca and liver—called also enterohepatitis, histomoniasis, infectious enterohepatitis
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