noun
1. : one of two or more persons acting in concert in the commission of a tort
2.
a. : one of two or more persons who may be joined as defendants in the same single cause of action to recover damage for a tort
b. : one of two or more persons jointly and severally fully responsible for all the damage to a third person caused by their concurring though independent tortious acts
c. : one of two tortfeasors where one is by the policy of the law vicariously liable for the tortious conduct of the other (as where the principal is liable for his agent's conduct or the master for that of his servant)