It is called either a "complaint" or a "petition". The nomenclature depends upon the jurisdiction and/or the underlying cause of action.
If you are filing the civil lawsuit against another party, you are filing a complaint.
A plaintiff initiates a lawsuit against a defendant.
A 'complaint' is the initial pleading that starts a civil action. It states the basis of the court's jurisdiction, the basis for the claim and the relief demanded by the plaintiff.
This question is vague, because "pleading" means many things. It could be the actual document you file, or it could be a specific statement in that document that you plead. Assuming you want to know what the pleading is called, the answer depends on jurisdiction. In Federal courts, for example, the initial pleading of a plaintiff is called a "complaint." In states that closely follow the Federal Rules of Civil Procedure, the word complaint is usually used to describe this pleading. In other state courts, the initial pleading of the plaintiff is known as a "petition," as in a petition to the court for an award of damages. If the question more generally asks what "first pleading of the plaintiff in a civil action" means, you can understand it as the first document filed by the plaintiff that initiates a lawsuit. In it the plaintiff identifies himself and his opponents, the defendants; sets out why jurisdiction and venue are proper; lays out his cause(s) of action against those defendants; and describes the damages he seeks. Different jurisdictions have different requirements on how specific this needs to be.
Defendant. He has to defend him self against the plaintiff's accusation.
The PLAINTIFF is the party who files (brings) the lawsuit.
The aggrieved party in a lawsuit is the plaintiff
If you have been served with a civil lawsuit, you will need to file a pleading called an Answer, where you answer each individual allegation, plead any affirmative defenses, and assert any counterclaims. A letter is not sufficient, and will not avoid a default judgment.
In civil law, the party who is served with the papers beginning the civil action, and is defending the lawsuit is called the "defendant"; the party who brings the lawsuit is called the "plaintiff". In the strictest sense, the person in a criminal 'lawsuit' not a civil proceeding, is called a "defendant". The party, in the criminal action, who brings the lawsuit, a criminal proceeding, is called the "State". The representative for the criminal action against the is a Prosecutor. Please see the discussion page for further notions about using the word "defendant" instead of the label "accused" in a criminal proceeding.
The person that files a civil suit is known as the Petitioner or the Plaintiff or the Claimant.
plaintiff
In a criminal case it is the accuser. In a civil case - think Judge Judy - it is the Plaintiff
Plaintiff