The person, company or organization who answers or defends a legal action taken by a plaintiff and against whom the court has been asked to order damages or specific corrective action to redress some type of unlawful or improper action alleged by the plaintiff.
Some legal disputes are taken by way of a petition in which case the initiator is called a "petitioner" and the defending party, the respondent.
Generally, however, the defendant starts his participation in a trial by responding to a statement of claim by way of a statement of defence and, if the facts lend themselves to it, a counterclaim.