Which document filed by the plaintiff outlines the basis of the complaint against the defendant?

The document that outlines the basis of the complaint against the defendant is referred to as a petition. In the context of civil litigation, a petition is a formal written request that initiates legal proceedings. It details the plaintiff's claims, the legal grounds for those claims, and the relief sought from the court. This document serves to inform the defendant of the charges against them and allows for the legal process to commence.

While a brief is generally a written argument submitted to the court, summarizing the case and providing legal analysis, it does not precisely outline the initial claims against the defendant. Pleading is a broader term that encompasses various documents filed in court, including both petitions and responses; however, it is less specific than the term petition in this context. An affidavit is a sworn statement of fact made under oath, often used to support motions or claims but not as the primary document outlining the complaint itself. Thus, the petition is the correct choice for detailing the basis of the complaint and initiating the civil process.