1. Any of various gram-negative, coccoid microorganisms of the genus Chlamydia, especially C. psittaci and C. trachomatis, that are pathogenic to humans and other animals, causing conjunctivitis in cattle and sheep and trachoma, urethritis, and proctitis in humans.
2. Any of several common, often asymptomatic, sexually transmitted diseases caused by the microorganism Chlamydia trachomatis.

[New Latin, genus name, from Latin chlamys, chlamyd-, mantle. See chlamys.]

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.