Mug shots: photographs of an accused taken at the time of his arrest, usually both face on and in profile.

I think I have finally answered the niggling question of the origin of the term. "Mug," of course, is a slang term for "face," and so I surmise that "mug shot" merely means "face shot." But only recently did I learn the connection between a drinking mug and the face. According to Webb Garrison in Why You Say It, beer mugs of the late 18th century were often shaped like human heads, and a not especially attractive person often bore a resemblance to a face on a mug. As a result, a face came to be called a mug.

I have yet to encounter a formal term which is the equivalent of "mug shot." Does anyone know one? I struggle to avoid slang terms in my formal writing, and am often wishing for a pithy substitute for "mug shot."