A ninja was a covert agent in feudal Japan, trained in espionage, sabotage, and unconventional warfare. While samurai fought openly with strict codes of honor, ninjas worked in secret, gathering intelligence, infiltrating enemy castles, and carrying out missions that required stealth rather than direct combat. They dressed in dark clothing to move unseen at night, knew how to pick locks and scale walls, and studied the art of misdirection and disguise.

Ninjas appear frequently in movies and video games, but many popular beliefs about them are myths or exaggerations. They probably didn't wear all black (which would stand out on moonlit nights), couldn't actually turn invisible, and rarely threw shuriken (the star-shaped weapons you see in films). Real ninjas were more like spies than superhuman warriors.

Today, when someone moves like a ninja, they're being extremely quiet and sneaky, perhaps sneaking downstairs for a midnight snack without waking anyone. You might call someone a stealth ninja if they're impressively good at moving without being noticed. The word has also been borrowed for other uses: a code ninja is a skilled computer programmer, for example. These modern uses capture the ninja's legendary skill and cleverness, even if they have little to do with feudal Japan.