Definition - What does Complainant mean?
A complainant is a person who begins a lawsuit by lodging a formal complaint against another person or a group with law enforcement or the appropriate court. Complainants are also known as plaintiffs. An example of a complainant would be a person who files a police report stating that they have been a victim of armed robbery. Another example is someone who files a petition with the court to sue another person.
Justipedia explains Complainant
Basically, a complainant is just the person who initiates a report or a lawsuit. People become complainants for a wide variety of perceived infractions against their rights. For example, a person may complain to the courts that they have been assaulted, that they have had their intellectual property infringed upon, that their property has been stolen, that their spouse did not pay child support, etc. In cases where the defendant is being prosecuted by the state, the state becomes the complainant because the defendant is accused of violating government laws.