An crime as well as an intentional tort. A a person commits false imprisonment when he commits an act of restraint on another person which confines that person in a bounded area. An act of restraint can be a physical barrier (such as a locked door), the use of physical force to restrain, a failure to release, or an invalid use of legal authority. Threats of immediate physical force are also sufficient to be acts of restraint. An area is only bounded if freedom of movement is limited in all directions. If there is a reasonable means of escape from the area, the area is not bounded.
An example of an invalid use of legal authority is the detainment or arrest of a person without a warrant, with an illegal warrant, or with a warrant illegally executed. So long as the person is deprived of his personal liberty, the amount of time actually detained is inconsequential. See, e.g. Schenck v. Pro Choice Network, 519 U.S. 357 (1997)