The person who starts, brings an case to court; who sues or who asks the Court for a trial and, if successful, enforceable relief.
In Re Shirleys Limited, (1916, Saskatchewan) 10 WWR 918:
"Plaintiff means a person who commences an action."
May also be called "claimant", "petitioner" or "applicant".
The person being sued is generally called the defendant or the respondent.
A plaintiff commences the pleadings of a Court action by writing, filing and delivery of a statement of claim.