A ninja (忍者) or shinobi (忍び) was a covert agent or mercenary 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 in certain situations.  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. 

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Few written records exist to detail the activities of the ninja. The word shinobi did not exist to describe a ninja-like agent until the 15th century, and it is unlikely that spies and mercenaries prior to this time were seen as a specialized group. In the unrest of the Sengoku period (15th - 17th centuries), mercenaries and spies for hire arose out of the Iga and Kōga regions of Japan, and it is from these clans that much of later knowledge regarding the ninja is inferred. Following the unification of Japan under the Tokugawa shogunate, the ninja descended again into obscurity.  However, in the 17th and 18th centuries, manuals such as the Bansenshukai (1676) — often centered around Chinese military philosophy — appeared in significant numbers.  These writings revealed an assortment of philosophies, religious beliefs, their application in warfare, as well as the espionage techniques that form the basis of the ninja's art. The word ninjutsu would later come to describe a wide variety of practices related to the ninja.

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

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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  In historical documents, shinobi is almost always used.

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Kunoichi, meaning a female ninja, supposedly came from the characters くノ一 (pronounced ku, no and ichi), which make up the three strokes that form the kanji for "woman" (女).

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Despite many popular folktales, historical accounts of the ninja are scarce. Historian Stephen Turnbull asserts that the ninja were mostly recruited from the lower class of samurai, and in fact at some level, even if it was the bottom of the warrior class, the ninja was samurai, even if it was at least ashigaru (foot soldiers).  That is why little literary interest was taken in them.   Instead, war epics such as the Tale of Hōgen (Hōgen Monogatari) and the Tale of the Heike (Heike Monogatari) focus mainly on the aristocratic samurai, whose deeds were apparently more appealing to the audience.  Historian Kiyoshi Watatani states that the ninja were trained to be particularly secretive about their actions and existence

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"So-called ninjutsu techniques, in short are the skills of shinobi-no-jutsu / shinobijutsu, which have the aims of ensuring that one's opponent does not know of one's existence, and for which there was special training."

 

The skills required of the ninja has come to be known in modern times as ninjutsu, but it is unlikely they were previously named under a single discipline. Modern misconceptions have identified ninjutsu as a form of combat art, but historically, ninjutsu largely covered espionage and survival skills. Some lineage styles (ryūha) of ninjutsu such as Tomo-Ryu and Togakure-Ryu are claimed to be descended from historical practices.

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The ninja did not always work alone. Teamwork techniques exist: for example, in order to scale a wall, a group of ninja may carry each other on their backs, or provide a human platform to assist an individual in reaching greater heights.  The Mikawa Go Fudoki gives an account where a coordinated team of attackers used passwords to communicate. The account also gives a case of deception, where the attackers dressed in the same clothes as the defenders, causing much confusion.  When a retreat was needed during the Siege of Osaka, ninja were commanded to fire upon friendly troops from behind, causing the troops to charge backwards in order to attack a perceived enemy. This tactic was used again later on as a method of crowd dispersal.

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Most ninjutsu techniques recorded in scrolls and manuals revolve around ways to avoid detection, and methods of escape.  These techniques were loosely grouped under corresponding natural elements. Some examples are:

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• Hitsuke - The practice of distracting guards by starting a fire away from the ninja's planned point of entry. Falls under "fire techniques" (katon-no-jutsu).

• Tanuki-gakure - The practice of climbing a tree and camouflaging oneself within the foliage. Falls under "wood techniques" (mokuton-no-jutsu).

• Ukigusa-gakure - The practice of throwing duckweed over water in order to conceal underwater movement. Falls under "water techniques" (suiton-no-jutsu).

• Uzura-gakure - The practice of curling into a ball and remaining motionless in order to appear like a stone. Falls under "earth techniques" (doton-no-jutsu).