A 'ninja' or 'shinobi' was a covert agent or soldier of feudal Japan specializing in unorthodox arts of war. The functions of the ninja included espionage, sabotage, infiltration, spying, arson, assassination, as well as open combat. The origin of the ninja is obscure and difficult to determine, but can be surmised to be around the 14th century. However, the antecedents to the Ninja may have existed as early as the Heian and early Kamakura eras.

Ninja is the on'yomi reading of two kanji. In the native kun'yomi reading, it is read shinobi, a shortened form of the longer transcription 'shinobi-no-mono' . The term 'shinobi' has been traced as far back as the late 8th century to poems in the Man'yoshu. The underlying connotation of 'shinobi' means "to steal away" and — by extension — "to forbear", hence its association with stealth and invisibility. 'Mono' means "a person".

Historically, the word ninja was not in common use, and a variety of regional colloquialisms evolved to describe what would later be dubbed ninjas. Along with 'shinobi', some examples include monomi ("one who sees"), nokizaru ("macaque on the roof"), rappa ("ruffian"), kusa ("grass") and Iga-mono ("one from Iga"). In historical documents, shinobi is almost always used.

Kunoichi, meaning a female ninja, came from the characters which make up the three strokes that form the kanji for "woman".

Similarly to the word 'budo', 'bujutsu; is a compound of the roots bu, meaning "martial", and jutsu , meaning "technique" Thus, budo is most often translated as "the way of war", or "martial way", while 'bujutsu' is translated as "science of war" or "martial craft." However, both budo and 'bujutsu' are commonly used interchangeably in English with the term "martial arts". 'Budo' and 'bujutsu' have quite a delicate difference; whereas bujutsu only gives attention to the physical part of fighting (how to best defeat an enemy), budo also gives attention to the mind and how one should develop oneself. Classical budo uses aspects of the lifestyle of the samurai of feudal Japan and translates them to self-development in modern life.