"Sex act" in which one or both partners lick and kiss the exterior of the other's anus, and penetrate the anal sphincter with the tongue.
Colloquially known as "rimming."
According to Dr. Jeffrey Klausner, director of STD Prevention and Control Services at the San Francisco Department of Public Health, oral-anal "sex," while safe for HIV, can transmit intestinal parasites, including shigella, amoeba, and giardia; hepatitis A; salmonella, cryptosporidium, and microsporidia; and perhaps syphilis and gonorrhea.
The parasitic diseases are notoriously difficult to treat; and all of the oral-anally vectored infections can cause both acute and chronic distress.
See also kink, fisting, and pansexualism.