Hiden (秘伝, Literally meaning: secret tradition) or simply "secret" techniques are passed down orally from generation to generation in certain regions or clans.
The groups who possess these techniques are usually extremely secretive about them in order to ensure that no one other than the members of the group or clan learn them. As such, they are passed down through oral means instead of being written down or recorded. A few hiden techniques require a special ability or lineage and as such are usually the pride and fame of the clan that possesses them. The Nara clan's Shadow Imitation Technique and the Aburame clan's Parasitic Destruction Insect Technique are examples of this. These techniques also cannot be copied through any means of simple observation.
They are not to be confused with kekkei genkai, as technically, hiden techniques can be taught to anyone.