What is Constituent?

Legal Definition
He who gives authority to another to act for him. 1 Bouv. Inst. n. 893.

2. The constituent is bound with whatever his attorney does by virtue of his authority. The electors of a member of the legislature are his constituents, to whom he is responsible for his legislative acts.
-- Bouviers Law Dictionary
Legal Definition
A word used as a correlative to "attorney," to denote one who constitutes another his agent or invests the other with authority to act for him. It is also used in the language of politics, as a correlative to "representative," the constituents of a legislator being those whom he represents and whose interests he is to care for in public affairs; usually the electors of his district
-- Black's Law Dictionary
Legal Definition
An agent’s principal.
-- Ballentine's Law Dictionary