n pl :  a family of bacteria (order Chlamydiales) that are related to members of the order Rickettsiales, that are obligate parasites in the cells of warm-blooded vertebrates, and that include the causative agents of trachoma, lymphogranuloma venereum, and psittacosis .
 
 
 
Published under license with Merriam-Webster, Incorporated.  © 1997-2004.