In civil procedure, the prosecuting party (the one filing a complaint or lawsuit or petition) is referred to as a “plaintiff” or “petitioner” or “complainant” (depending upon the court and the nature of the matter), while the opposing party is referred to as a “defendant” or “respondent.” (For purposes of simplicity, the terms “plaintiff” and “defendant” are used exclusively herein, but imply any or all of the above, respectively.)
Any person may file a lawsuit under his or her own name, but the person must have “legal capacity” to sue (the legal competency to stand before the court). This requirement implies, among other factors, minimum legal age and mental competency. FRCP 17(c) provides that a guardian or conservator may sue or defend on behalf of an infant or legally incompetent person; or, if none exists, the court will appoint a “next friend” or “guardian ad litem” to represent the interest of the child or incompetent person. A deceased person may be represented in an action by the personal representative (executor or administrator) of the deceased’s estate. FRCP 17(b) also provides that in federal court, the legal capacity of a business corporation to sue or be sued is determined by the law under which it was organized.
Several parties may be joined in an action, as co-plaintiffs or co-defendants. Under FRCP 23 and most state rules, multiple plaintiffs who have suffered harm as a result of the actions of a common defendant may be joined together in one lawsuit called a “class action.” Under such a suit, only a few plaintiffs will be named in the action, but they will represent all plaintiffs within the certified “class,” and their claims must be fairly representative of the interests of all the persons within the class.
A lawsuit may become fairly complicated when the original parties (and sometimes the court) bring in third or additional parties not initially named in the suit. Parties joined on the same side are referred to as “co-parties.” If co-parties raise claims against one another (e.g., a defendant blames another defendant), they are “cross-parties” as to each other. But if a “counter-claim” is raised against an opposing party, they become “counter-parties” as to the counterclaim. In the “caption,” or heading of the original action, the parties may be referred to as co-plaintiffs, co-defendants, cross-plaintiffs, cross-defendants, counter-plaintiffs, counter-defendants, or “interested parties,” depending upon the claims or defenses raised.