What is Actor?

Legal Definition
In policy debate, an actor is an entity that enacts a certain policy action. If a plan were to have the U.S. send humanitarian aid to Sudan, then the actor would be the United States federal government.

Many times, actors are subdivided into more specific "agents".

The most common agents include the Supreme Court, the President (usually through the use of an Executive Order), and Congress. Sometimes, the actors get smaller and devolve into Executive agencies. For example, on a previous high school debate topic—the use of renewable energy—the plan could use the Department of Energy.

Sometimes the Negative will use a counterplan to solve for the harms of the affirmative and the most common method of doing so is by the use of an agent counterplan, which simply does the mandates of the Affirmative plan through the use of another agent. Sometimes, the Negative will even use another country. If the Affirmative plan were to send peacekeeping troops to Congo, then the Negative would have Bangladesh (or any other country), do it.

Theoretical debates often ensue as to the legitimacy of agent counterplans. For a video about agent counterplans, try [1]
-- Wikipedia
Legal Definition
Practice. 1. A plaintiff or complainant. 2. He on whom the burden of proof lies. In actions of replevin both parties are said to be actors. The proctor or advocate in the courts of the civil law, was called actor.
-- Bouviers Law Dictionary
Legal Definition
In Roman law. One who acted for another; one who attended to another's business; a manager or agent. A slave who attended to, transacted or superintended his master's business or affairs, received and paid out moneys, and kept accounts. Burrill. A plaintiff or complainant. In a civil or private action the plaintiff was often called by the Romans "petitor;" in a public action (causa publica) he was called "accusator." The defendant was called "reus," both in private and public causes; this term, however, according to Cicero, (De Orat. ii. 43,) might signify either party, as indeed we might conclude from the word itself.

In a private action, the defendant was often called "adversarius," but either party might be called so. Also, the term is used of a party who, for the time being, sustains the burden of proof, or has the initiative in the suit. In old European law. A proctor, advocate, or pleader; oue who acted for another in legal matters; one who represented a party and managed his cause. An attorney, bailiff, or steward; one who managed or acted for another. The Scotch "doer" is the literal translation.
-- Black's Law Dictionary
Legal Definition
A Roman law term denoting a pleader or advocate; the plaintiff in an action.
-- Ballentine's Law Dictionary