What is Board Of Supervisors?

Legal Definition
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).
-- Wikipedia
Legal Definition
Under the system obtaining in some of the northern states, this name is given to an organized committee, or body of officials, composed of delegates from the several townships in a county, constituting part of the county government, and having special charge of the revenues of the county.
-- Black's Law Dictionary
Legal Definition
The governing body of a county, in many states.
-- Ballentine's Law Dictionary