In Japanese history, a is a warrior, trained in martial arts, and specializing in a variety of unorthodox arts of war. The methods used by ninja included assassination, espionage, stealth, camouflage, specialized weapons, and a vast array of martial arts were added after the 20th century.
The exact origins are unknown, but their roles may have included sabotage, espionage, scouting and assassination missions as a way to destabilize and cause social chaos in enemy territory or against an opposing ruler, perhaps in the service of their feudal rulers (daimyo, shogun), or an underground ninja organization waging guerilla warfare.
The word ninja became popular in the post-World War II culture. The nin of ninjutsu is the same as that in ninja, whereas jutsu (術) 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."
Other terms which may be used include oniwaban (お庭番 "one in the garden"), suppa, rappa, mitsumono, kusa (草 grass) and Iga-mono ("one from Iga").
Ninja as a group first began to be written about in 15th century feudal Japan as martial organizations predominately in the regions of Iga and Koga of central Japan, though the practice of guerrilla warfare and undercover espionage operations goes back much further.
At this time, the conflicts between the clans of daimyo that controlled small regions of land had established guerrilla warfare and assassination as a valuable alternative to frontal assault. Since Bushidō, the samurai code, forbade such tactics as dishonorable, a daimyo could not expect his own troops to perform the tasks required; thus, he had to buy or broker the assistance of ninja to perform selective strikes, espionage, assassination, and infiltration of enemy strongholds. There are a few people and groups of people regarded as having been potential historical ninja from approximately the same time period.
Though typically classified as assassins, however in his book Mystic Arts of the Ninja Stephen K. Hayes depicts them in armour similar to a samurai. Hayes also says those who ended up recording the history of the ninja were typically those within positions of power in the military dictatorships. According Hayes and Masaaki Hatsumi
"Ninjutsu did not come into being as a specific well defined art in the first place, and many centuries passed before ninjutsu was established as an independent system of knowledge in its own right. Ninjutsu developed as a highly illegal counter culture to the ruling samurai elite, and for this reason alone, the origins of the art were shrouded by centuries of mystery, concealment, and deliberate confusion of history.
A similar account is given by Hayes: "The predecessors of Japan's ninja were so-called rebels favoring Buddhism who fled into the mountains near Kyoto as early as the 7th century A.D. to escape religious persecution and death at the hands of imperial forces.
Ninja museums in Japan declare women to have been ninjas as well. A female ninja may be kunoichi (くノ一); the characters are derived from the strokes that make up the kanji for female (女). They were sometimes depicted as spies who learned the secrets of an enemy by seduction; though it's just as likely they were employed as household servants, putting them in a position to overhear potentially valuable information.
As a martial organization, it has been assumed that ninja would have had many rules, and keeping secret the ninja's clan and the daimyo who gave them their orders would have been one of the most important ones. For modern hierarchy in ninjutsu, see the article about ninjutsu.
The stereotypical ninja that continually wears easily identifiable black outfits (shinobi shozoku) comes from the kabuki theater. Prop handlers would dress in black and move props around on the stage. The audience would obviously see the prop handlers, but would pretend they were invisible. Building on suspension of disbelief, ninja characters also came to be portrayed in the theatre as wearing similar all-black suits. This either implied to the audience the ninja were also invisible, or simply made the audience unable to tell a ninja character from many prop handlers until the ninja character distinguished himself from the other stagehands with a scripted attack or assassination.
Boots that ninja used (jika-tabi), like much of the rest of Japanese footwear from the time, have a split-toe design that improves gripping and wall/rope climbing. They are soft enough to be virtually silent. Ninja also attached special spikes to the bottoms of the boots called ashiko.
The actual head covering suggested by Masaaki Hatsumi in his book The Way of the Ninja: Secret Techniques utilizes what is referred to as sanjaku-tenugui, three-foot cloths. It involves the tying of two three-foot cloths around the head in such a way as to make the mask flexible in configuration but securely bound. Some wear a long robe, most of the time dark blue (紺色 kon'iro) for stealth.
Ninja also employed a variety of weapons and tricks using gunpowder. Smoke bombs and firecrackers were widely used to aid an escape or create a diversion for an attack. They used timed fuses to delay explosions. Ōzutsu (cannons) they constructed could be used to launch fiery sparks as well as projectiles at a target. Small "bombs" called metsubushi (目潰し, "eye closers") were filled with sand and sometimes metal dust. This sand would be carried in bamboo segments or in hollowed eggs and thrown at someone, the shell would crack, and the assailant would be blinded. Even land mines were constructed to use a mechanical fuse or a lit, oil-soaked string. Secrets of making desirable mixes of gunpowder were strictly guarded in many ninja clans.
Other forms of trickery were said to be used for escaping and combat. Ashiaro are wooden pads attached to the ninja's tabi (thick socks with a separate "toe" for bigger toe; used with sandals). The ashiaro would be carved to look like an animal's paw, or a child's foot, allowing the ninja to leave tracks that most likely would not be noticed.
A small ring worn on a ninja's finger called a shobo would be used for hand-to-hand combat. The shobo would have a small notch of wood used to hit assailant's pressure points for sharp pain, sometimes causing temporary paralysis. A suntetsu is very similar to a shobo. It could be a small oval shaped piece of wood affixed to the finger by a small strap. The suntetsu would be held against a finger on the palm-side and when the hand was thrust at an opponent using the longer piece of wood to target pressure points such as the solar plexus.
Some believe ninja used special short swords called ninjato, or shinobigatana. Ninjato are smaller than katana but larger than wakizashi. The ninjato was often more of a utilitarian tool than a weapon, not having the complex heat treatment of a usual weapon, and a straight blade. It should be noted there have been no actual Ninjato found, and their existence is purely speculative. In all probability, ninja used the standard swords of the time. Another version of the ninja sword was the shikoro ken (saw sword). The shikoro ken was said to be used to gain entry into buildings, and could also have a double use by cutting (or slashing in this case) opponents.
The shuriken is a weapon that was barely ever used for throwing. It would be stuck into a wall or the ground to be used as a distraction, similar to Caltrops. Shuriken were often used coated with poison so when in direct combat with another the ninja could throw the shuriken and have a more substantial effect than the minor physical injury (with potentially severe effects depending on the strength of the poison). Shuriken does not actually refer to a singular weapon, in actuality the word refers to the general group of a ninja's throwing weapons ie; shaken and kunai and various sharpened conical or spike-shaped pieces of metal.
Many ninja disguised themselves as farmers so their weapons (the kama, for example) could double as both weapons and farming implements.
Ninja appear in both Japanese and Western fiction. Depictions range from realistic to the fantastically exaggerated with sources, including books, television, movies, videogames and websites portray ninja in non-factual ways, often for humor or entertainment.