noun
A historical covert agent in feudal Japan trained in ninjutsu. In modern usage, often refers to the pop-culture figure.
See also: 忍術
忍者は忍術を使って敵地に潜入した。
Ninjas infiltrated enemy territory using ninjutsu.
あのアニメの主人
The main character of that anime is a ninja.
忍術 refers to the art or techniques practiced by ninjas, while 忍者 refers to the person who practices them.
From 忍 (nin, 'to endure, conceal') + 者 (sha, 'person'). The reading にんじゃ is a regular on'yomi combination.