Posse comitatus

[Sarah]

The power of the county, or the citizens who may be summoned bythe sheriff to assist the authorities in suppressing a riot, orexecuting any legal precept which is forcibly opposed. Blackstone.

[Dave Paprocki]

In English [common law], the power of a county [sheriff] to enlist able-bodied citizens of the county to help him in the pursuit and arrest of a [felon].