The people we call ninja mostly did not call themselves that.

The word is Japanese 忍者. Its first character, 忍, can be read nin or, in the native reading, shinobi. Merriam-Webster glosses nin as covering perseverance, concealment and stealthy movement. The second character, 者 (ja), means "person." So a ninja is literally a person of stealth, or of endurance.

The historical term was shinobi, often in the phrase shinobi no mono. The International Ninja Research Center at Mie University says "ninja" is a modern label for those historically called shinobi. Military History Now says the term 忍者 does not actually appear until the 20th century. It also notes the two characters were in use from at least the late 1500s. Nippon.com says "ninja" became the common term only in the mid-twentieth century. So the exact date of first use is unsettled, but the common label is modern.

The agents themselves were real. Mie University dates shinobi activity from the 14th century to the second half of the 19th, and traces their roots to bands called akuto. These gained military influence late in the Kamakura period. Nippon.com describes them as covert agents who gathered intelligence and carried out sabotage behind enemy lines, with the earliest record in the 14th-century Taiheiki.

Much of the popular image is later. According to Military History Now, black outfits appear in the record only after 1600, and surviving manuals describe ordinary clothing, with white for moonlit nights and black for dark ones. Sources differ on throwing stars: Military History Now finds just one in the surviving documents, while Nippon.com says there is no record of shinobi using them. The claim that they never wrote anything down is also wrong. Military History Now's educated guess is about 500 original documents, including the Bansenshukai, a massive work that Nippon.com says was compiled in 1676 in 22 volumes. Edo-period fiction then built the legend further.

English picked the word up late. Merriam-Webster's first known use is from 1964, for a person trained in ancient Japanese martial arts and employed for espionage and assassination. Films and books carried it into everyday English, where it now also means anyone unusually skilled at something.