Plaintiff
Etymology
From Old French plaintif, "complaining, aggrieved," from plainte, "complaint, lamentation," from Latin planctus, "a beating of the breast in grief," from plangere, "to beat, to lament." The plaintiff is literally the one voicing a grievance. The same root gives us plaintive, the mournful cousin of this dry legal term.
noun · /PLAYN-tif/
The person who brings a lawsuit against someone else in a civil court.
What it names
Whoever’s side comes first in a case caption — the party doing the suing, as opposed to the one being sued.
Example
As plaintiff, she had to prove the company’s product had actually caused her illness.
Related idea
The plaintiff starts the action; the defendant is the party answering it. In a criminal case the roles are prosecution and defendant instead.