Constituent

n.
  1. The person or thing which constitutes, determines, or constructs.
  2. That which constitutes or composes, as a part, or an essential part; a component; an element.
  3. One for whom another acts; especially, one who is represented by another in a legislative assembly; -- correlative to representative.
  4. (Law) A person who appoints another to act for him as attorney in fact.