Three strikes laws are statutes enacte by state governments in the United States which require the state courts to hand down a mandatory and extended period of incarceration to persons who have been convicted of a serious criminal offense on three or more separate occasions. These statutes became very popular in the 1990s. They are formally known among lawyers and law professors as habitual offender laws. The name comes from baseball, where a batter has two strikes before striking out on the third.