A mug shot is a photographic portrait taken soon after one is arrested. Most mug shots are two-part, with one side-view photo, and one front-view.
Prior to the advent of computer technology, the accused was asked to hold a card with their name, the date, and other information on it.
In recent years, digital photography is used for the booking process, and the accused now is no longer asked to hold the card while the photo is taken. Rather the digital photograph is linked to a database record concerning the arrest.
The purpose of the mug shot is to allow law enforcement to have a photographic record of all arrested individuals to allow for identification by victims and investigators. Such photograph(s) are automatically entered into the public domain in the United States, and can be obtained by anyone through the Freedom of Information Act, except for when the arrestees' record has been sealed.
The term derives from mug, an English slang term for face, dating from the 18th century. Another source suggests the term comes from mug, as in grimace, because early subjects would try to reduce their mugshot's value for later identification by grimacing or otherwise twisting their facial muscles (mugging).