- a member of a feudal Japanese society of mercenary agents highly trained in martial arts and stealth (ninjutsu ), who were hired for covert purposes ranging from espionage to sabotage and assassination.
- a person who is expert or highly skilled in a specified field or activity (often used attributively): marketing ninjas; ninja programmers.
- (in a video game) to unfairly claim (valuable items or loot) from a treasure pool by taking them from a kill made by another player or bypassing the agreed-upon system for loot distribution in a group: When I ninja an epic item, I log off the server before anyone even realizes what happened.
- a person skilled in ninjutsu, a Japanese martial art characterized by stealthy movement and camouflage
