From Academic Kids
A constituent is someone who can or does appoint or elect (and often by implication can also remove or recall) another as her agent or representative. A constituency is all the constituents of a particular agent or representative.
Depending on the context, another closely-related meaning of constituent is that of a citizen residing in the area governed, represented, or otherwise served by a politician; sometimes this is restricted to citizens who elected the politician. All the constituents who are registered to vote are referred to as the electorate.
Constituent service is representatives helping their constituents handle problems that can be best handled by a representative getting involved. Members of the U.S. Congress (both Representatives and Senators) working in Washington, D.C. have a governmentally staffed district office to aid in constituent service. Many state legislatures have followed suit.
Elected officials tend to believe that responsiveness to their constituents is their highest professional responsibility. A longtime Pennsylvania elected official, State Rep. Mark B. Cohen of Philadelphia, said that "Opinions about political and governmental issues are always appropriate to share with elected officials. All too often, the gap between what people think and what they express is very wide, creating a vacuum filled of interests not representative of the general public."
In a looser sense, corporations and other such organizations can be referred to as constituents, if they have a significant presence in an area.