A complaint where the plaintiff (or, in limited cases, the plaintiff's counsel) swears to the allegations, demonstrating to a court that the plaintiff has investigated the charges against the defendant and found them to be of substance. In many jurisdictions, a complaint does not need to be verified unless a rule or statute specifically states otherwise.
Typically a plaintiff verifies a complaint by attaching a page at the end containing a statement made under oath that:
The plaintiff has reviewed the complaint.
Regarding the allegations of which the plaintiff has personal knowledge, the plaintiff knows or believes them to be true.
Regarding the allegations of which the plaintiff does not have personal knowledge, the plaintiff believes them to be true based on specified information, documents, or both.