Glossary
Plaintiff
The person or entity who starts a civil lawsuit by filing a complaint. The plaintiff is the party who claims to have been harmed and is asking the court for some kind of remedy.
In a civil case, the plaintiff is the party that goes to the court and asks it to do something: order the defendant to pay money, stop a behavior, return property, or grant some other remedy. The plaintiff carries the burden of proof, meaning it's their job to show the court that their version of what happened is more likely true than not.
In some kinds of cases: especially family law, probate, or appeals: the plaintiff is called a petitioner instead. The role is the same: the one bringing the case.