A defendant is one who responds and defends a court action initiated by a 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.
In a tort or personal injury action, the defendant is always the person whom, it is alleged, is the tort-feasor; the plaintiff being the alleged injured party.
In a third party claim, it is the defendant who seeks to draw in a third party.