a government official who conducts criminal prosecutions on behalf of the state
"The prosecutor argued that the evidence was sufficient to secure a conviction against the defendant."
a prosecuting attorney.
"The prosecutor argued that the evidence clearly showed the defendant's guilt."
The word entered English in the 1590s from Medieval Latin prosecutor, which originally meant one who pursues a case forward. While it looks like a simple combination of "prosecute" and "-or," its roots actually trace back to the verb meaning "to follow up on or pursue."