Kinjutsu (禁術; Literally meaning "Forbidden Techniques") are techniques that have been banned from being taught or used. This ban could have been put in place for any number of reasons, but kinjutsu can generally be put in either of three categories:
Techniques that cause harm to the user themselves, such as opening the Eight Gates, which the mere use of is both highly useful as well as detrimental to the user. Techniques that violate the laws of nature (e.g. the Summoning: Impure World Reincarnation, which reincarnates the dead with a human sacrifice). Certain techniques that are known to cause massive collateral damage, such as the total destruction of a village and end with the death of everyone in it, thus the great moral ramifications of its potential lead many to labelling it as a forbidden technique. Despite being banned, the use of these techniques generally does not carry any legal consequences in respects to the village they're used in, except those brought on by the techniques themselves. However, their use is heavily frowned upon, especially those techniques that violate nature.