The Online Asbestos, Asbestos Cancer, & Mesothelioma Authority
Complainant
This is basically a synonym for plaintiff or claimant. The complainant is the party that initiates the lawsuit, or in the case of criminal proceedings, the district attorney who files charges on behalf of the state.
In civil cases, the complainant is seeking a legal remedy, or redress, for injury or loss. By successfully arguing his/her case before a judge and jury (except in the case of a bench trial, in which there is no jury) and supporting those arguments with valid evidence, the complainant will be receive a judgment in his/her favor. The judge will issue a court order requiring the defendant to pay an amount of money to the plaintiff/complainant in compensation for expenses related to the injury as well as lost income and compensation for pain and suffering.
In most of the U.S. , a civil suit is initiated by what is known as service of legal process. This means that an officer of the court, known as a process server, must deliver documents outlining the complaint to the defendant in person and physically place it in his/her hands. Afterwards, these documents are filed with the court, accompanied by an affidavit - a paper on which the process server has "affirmed" that s/he has delivered said documents in accordance with the rules of civil procedure.
Not all litigation calls for a jury or bench trial. Sometimes, the procedure is to hold an informal hearing. If this is the case, the plaintiff is then known as a petitioner, while the defendant is called the respondent.
There are also civil actions that are not adversarial - in other words, do not involve one side against another. These are actions that involve a petitioner or petitioners who are seeking some specific statutory relief that may require a judge's approval; an example of this might be a developer who is attempting to get an exemption from a zoning law.