Русско-англо-русский юридический словарь

A defendant is one who responds and defends a court action initiated by a plaintiff. (Duhaime's Law Dictionary)

The US energy regulator has taken civil action against a Dutch-based oil trading firm and three top employees for manipulating crude oil prices. [...] Bastiaan van Kempen, the boss of Chicago-based Optiver, a subsidiary of Optiver Holding, was named as a defendant along with trader Christopher Dowson and Randal Meijer, a supervisor of Optiver. (BBC News)

The person who commences civil action is usually called "plaintiff". The party that is sued is called "defendant". (torontolitigationlawyer.ca)

The defendant in a lawsuit is the person against whom the action is brought, by the plaintiff. A defendant in an arbitration case or a divorce case is called the "respondent." (The Balance Small Business)

In Scottish law a plaintiff is referred to as a "pursuer" and a defendant as a "defender". (Wikipedia)