The wrongful, intentional restraint of another person without the legal right to do so. False imprisonment can involve actual physical restraint, such as locking someone in a car or tying a person to a chair. But it's not necessary that physical force be used. Threats or a show of apparent authority are sufficient.
False imprisonment can be both a misdemeanor (a crime that results in jail time) and a tort (a civil wrong that leads to damages liability). If the perpetrator confines the victim for a substantial period of time (or moves the victim a significant distance) in order to commit a felony, the false imprisonment may become a kidnapping.
People who are arrested and get the charges dropped, or are later acquitted, often think that they can sue the arresting officer for false arrest, a kind of false imprisonment. These lawsuits rarely succeed: As long as the officer had probable cause to arrest the person, the officer will not be liable for a false arrest, even if it turns out later that the information the officer relied upon was incorrect.