Wiktionary
n. (context US English) The elected governing body of a county in certain states, which often has executive as well as legislative powers
Wikipedia
A board of supervisors is a governing body that oversees the operation of county government in all U.S. counties in Arizona, California, Iowa, Mississippi, Virginia, and Wisconsin as well as 16 counties in New York. There are equivalent agencies in other states.
Similar to a city council, a board of supervisors has legislative, executive, and quasi-judicial powers. The important difference is that a county is an administrative division of a state, whereas a city is a municipal corporation; thus, counties implement and, as necessary, refine the local application of state law and public policy, while cities produce and implement their own local laws and public policy (subject to the overriding authority of state law).
Usage examples of "board of supervisors".
Hal Cross is the deputy in charge of prosecuting Kailor, who's charged with killing Grossman, who was chairman of the board of supervisors.
County funds had been slashed at the last board of supervisors meeting, and since Juniper's library was the smallest and least frequented in the county, the decision had been made to close it.
In less than two weeks time the board of supervisors is going to decide whether or nor to accept the health department's restructuring plans, plans which stave off the worst of the cuts in return for a leaner system long-term.
I railed against closed meetings held by the city council and the county Board of Supervisors.