忍者, the Japanese ninja symbol (kanji).

A Ninja (忍者; plural ninja or ninjas) is one who trains in the art of Ninjutsu. They were originally assassins and spies in feudal Japan.  A male ninja is called a shinobi (忍び), while a female ninja is called a kunoichi (くノ一).

Ninjas in the TMNT Universe

Accuracy of terms

While within the confines of the show and many other media, historically the word shinobi is not a descriptor for simply male ninja, but a gender neutral. The word Shinobi is gender neutral, meaning practitioner of the ninjutsu. Kunoichi does also not necessarily refer to female ninja, but ninja trained in the art of seduction to achieve their goals. Most of them were female, but it has been stated that men could learn their seductive arts if they chose to.

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