Posse started out as a technical term in law, part of the term "posse comitatus," which in Medieval Latin meant power or authority of the county. As such, it referred to a group of citizens summoned by a sheriff to preserve the public peace as allowed for by law. "Preserving the public peace" so often meant hunting down a supposed criminal that "posse" eventually came to mean any group organized to make a search or embark on a mission. In even broader use it can refer to any group, period. Sometimes nowadays that group is a gang or a rock band but it can as easily be any group - of politicians, models, architects, tourists, children, or what have you - acting in concert.