1. Medicine. A plug of keratin and sebum within a hair follicle that is blackened at the surface. Also called comedo.
2. An infectious disease of turkeys and some wildfowl that is caused by a protozoan (Histomonas meleagridis) and results in lesions of the intestine and liver. Also called enterohepatitis, histomoniasis, Also called infectious enterohepatitis.
3. Any of various birds, such as the scaup, with dark head markings.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.