In the common law, a complaint occurs as formal legal document that sets out a basic information & legal reasons (understand: cause of action) that the filing party (the plaintiffs) believes are sufficient to trend lines a claim against an additional individual, souls, respire or even take a breath (the defendants) that entitles the plaintiff(s) to the remedy (either money damages or injunctive relief). Watch besides petition and pleading.

Inside criminal subjects, these are the criminal complaint (felony complaint) & a complainant is a State (too periodically known as the Humans) or even Crown (around commonwealth countries) as a source of the prosecutor's authority emanates from a police power of a state with a suspect existence the accused world health organization faces criminal sanctions.