This is the party against whom a claim is asserted. The party being sued either in tort, contract or otherwise. The term defendant is derived from the word “defend”. The defendant is thought of as the person charged with a crime. He is defending the criminal charge brought by the government.
However the term also applies in the civil context to a party who is being sued. The attorney who represents the defendant is referred to as defense counsel. Also the term “defense” may apply broadly to the team of lawyers representing that party. In addition the term may apply to specific theories that are being used to promote the interest of that party.
For more info on this term see the pages on Wikipedia.