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Ninja is the on’yomi reading of the two kanji used to write shinobi-no-mono, one of the native Japanese words for people who practice ninjutsu (sometimes transliterated as ninjitsu).

Ninja and shinobi-no-mono, along with shinobi, another variant, became popular in the post-World War II culture. The term , has been traced as far back as Japan’s Asuka period (538-710 AD), when Prince Shotoku is alleged to have employed one of his retainers as a ninja.[citation needed] The underlying connotation of shinobi (pronounced nin in Sino-Japanese compounds) is "to do quietly" or "to do so as not to be perceived by others" and by extension" to forebear," hence its association with stealth and invisibility. Mono (likewise pronounced sha or ja) means "thing" and/or "person." The nin of ninjutsu is the same as that in ninja, whereas jutsu  means skill or art, so ninjutsu means "the skill of going unperceived" or "the art of stealth"; hence, ninja and shinobi-no-mono (as well as shinobi) may be translated as "one skilled in the art of stealth."

Similarly, the pre-war word ninjutsu-zukai means "one who uses the art of remaining unperceived."

*Meaning of Ninja taken from Wikipedia