Posse Comitatus Law and Legal Definition
Posse comitatus is the common-law authority of the county sheriff to conscript any able-bodied male older than a certain age to assist him in keeping the peace or to pursue and arrest a felon. A posse comitatus could be used to suppress civil disorder. It is the law enforcement equivalent of summoning the militia for military purposes.
The Posse Comitatus Act is a federal law passed in 1878 which limits the powers of the federal government to use the military for law enforcement. It generally prohibits federal military personnel and units of the National Guard under federal authority from acting in a law enforcement capacity within the United States, except where expressly authorized by the Constitution or Congress. The Coast Guard is exempt from the Act. Because of this act, the military may generally not be used to maintain "law and order" on non-federal property.