Secret Art

Any skill or ability that only a select few know how to/can use, from magic to martial arts to metallurgy techniques. Even the art itself might be unknown to the world at large, but the crux of the trope is that the know-how to use it is limited to a relatively isolated group or person. Some Secret Arts aren't even particularly jealously guarded and only remain secret via their sheer obscurity.

Sometimes the technique is similar to Lost Technology being a Supernatural Martial Arts techniques, magical spells or just bread-making techniques that were invented eons ago but have been lost/buried/forgotten/not quite destroyed for a variety of reasons.

Perhaps their civilization collapsed or these techniques were simply were too powerful and/or dangerous and needed to be sealed away the point is no one has seen and/or heard about them in a very long time. They will always however be superior to their modern counterpart which will often make them the subject of a lengthy search quest by heroes or villains to further the plot.

Usually, one of the benefits of joining The Order. Can be (or encompass) the Dangerous Forbidden Technique. The Dark Arts are often this due to being outlawed. Often taught by a type I or III Obsolete Mentor.

Compare Supernatural Martial Arts, Super Empowering, and The Chosen Many. Contrast Everyone Is a Super.

Anime

 * Triangle Heart 3: The Fuwa-ryuu sword style, exclusive to the Takamachi family.
 * Naruto has a few forbidden Jutsu sealed away in scrolls kept heavily guarded due to them being either Dangerous Forbidden Techiniques or Dark Arts.
 * Yu-Gi-Oh the Abridged Series' explanation for why nobody could summon Exodia until Yami did: "This game makes no sense and nobody could figure out how to do it!"
 * The Hokuto no Ken in Fist of the North Star, a series of seven punches that results in the instant death of the opponent, is known only to Kenshiro and Raoh.
 * This is actually enforced by the practitioners of the style, as Hokuto Shinken is only allowed to have one successor.
 * Rokushiki from One Piece.

Film

 * Star Wars: The Force, though the Star Wars Expanded Universe shows that the Jedi Order doesn't have quite as strict a monopoly on Force abilities as they'd perhaps like.
 * Five Point Palm Exploding Heart Technique from Kill Bill which even Bill was surprised was taught to someone else by Pai Mei.

Literature

 * The Destroyer: Sinanju. Only two people in the world know it.
 * Within the first hundred or so books or so (literally; its a long series), we've run across a few others. In the history of the Korean village the art's home to, there have been a few rogue students and at least one second son who received surreptitious training and chose self-exile. In fact, Sinanju actually turns out to be in Remo's genetic heritage... but still, it's a pretty well-kept secret and there are probably at most three masters of it alive at any given time.
 * Discworld Of course the monks don't know it, he hasn't taught them!

Video Games

 * Dragon Age: The Templar's anti-magic abilities are exclusive to them among the Ferelden warriors. Also, Blood Magic.
 * The golem-making.
 * The Elder Scrolls: Necromancy tends to become this in times and places when it is illegal.
 * Street Fighter: The Ansatsuken fighting style
 * In Legend of Zelda: Twilight Princess (and possibly other Zelda games), Wolf Link can find a golden wolf spirit, which is actually an ancient warrior, who teaches him various forgotten sword techniques which greatly boost his effectiveness.
 * Somewhat toyed with concerning the Early Summer Rain Jab in the last case of Ace Attorney Investigations.

Western Animation

 * Avatar: The Last Airbender. In need of gaining firebending Aang and Zuko travel to the city of the first firebenders and discover ancient dragons who teach them an ancient form of firebending (or at least the philosophical underpinnings of it; fire as life and energy, rather than simply rage and destruction)

Real Life

 * Martial Arts: Many forms, such an Ninjitsu, were traditionally taught in secret by a very few masters. The fact that they were explicitly forbidden and/or illegal forced them to remain underground. Sometimes prospective students were bound by an oath of silence, especially when this was a weapon of rebels. These days, though, secret techniques don't remain secret for very long.
 * Some techniques were developed by particular martial artists, who died without passing them on. But those arts are not so much Secret as they are Forgotten.
 * Liechtenauer's school of German swordsmanship. In its time it was very exclusive, and it became entirely lost at some point in the 18th century. It's been brought back from the dead thanks to the finding of a handful of Medieval and Renaissance combat manuals, but it remains highly obscure.
 * In Vajrayana Buddhism and presumably many other religions, certain teachings are only passed down from master to student.
 * In the old days it was stage magic. In the 1800s, it was much harder to learn how to do magic even if would-be magicians were serious about learning because other magicians were so reluctant to teach newbies back then. Nowadays, there are hundreds of books available for learning the art--but even today it's still frowned upon by most magicians to reveal a secret to a layperson. Also, if one wishes to join a group such as the The Magic Castle or The Magic Circle, one usually has to prove that they're at least somewhat competent with magic by demonstrating decent ability and take an oath swearing that they will never reveal magic secrets to anyone not serious about learning the art--failure to follow this oath can get you banned from most groups