(USA) A crime of lesser seriousness than a felony where the punishment might be a fine or prison for less than one year.
The Office of the Attorney General for the State of California, in 2007, defined a felony as "a crime which may be punishable by imprisonment in a state prison and/or a fine, or death" and a misdemeanor, "a crime punishable by imprisonment in the county jail for not more than one year, by fine, or both."
While not an absolute definition as it varies from state to state, an example of a misdemeanor might be a theft of something of small value, disorderly conduct or a traffic violation, whereas a repeat misdemeanor may be upgraded to a felony (such as for DUI).