Venusian aikido

Venusian aikido (sometimes called Venusian karate) was an unarmed combat style used frequently by the Third Doctor to defend himself and others.

Nature
According to one account, it was created by the Venusians on Venus, which meant it worked best with five arms and five legs. (PROSE: First Frontier) Another account stated that the Doctor himself invented it. (PROSE: Interference - Book One)

It could be used to immobilise an opponent by using pressure points (TV: The Mind of Evil, Inferno) and perform joint locks, throws and kicks. (TV: The Claws of Axos) It tended to use the attacker's own movements against them. (PROSE: Freedom) These were effective against many different humanoid species, but were useless against non-human duplicates and seemed to have no effect on Ogrons or Sontarans. (TV: The Claws of Axos, Day of the Daleks, The Time Warrior)

The Doctor
The Second Doctor learned the arts of Venusian aikido whilst on Venus with Jamie McCrimmon and Victoria Waterfield. (AUDIO: Voyage to Venus)

The Third Doctor used it to disable Professor Stahlman. He claimed that, if the move was held for a period, it would paralyse the target permanently. (TV: Inferno)

The Third Doctor tried to use Venusian aikido against Primords. When this didn't work he moved on to Martian karate, but this had no effect either. (PROSE: Inferno)

The Third Doctor used his knowledge of Venusian martial arts to defend himself against attacking Uxarieans. (TV: Colony in Space)

The Third Doctor used Venusian Aikido to subdue Varan when he was attacked whilst trying to escape the Skybase (TV: The Mutants)

The Third Doctor also proved himself a formidable opponent using Venusian martial arts against several guards. (TV: The Green Death)

The Third Doctor also tried to use Venusian Aikido against Jingo Linx, but it proved to be ineffective. (TV: The Time Warrior)

The Third Doctor used Venusian Aikido against Remoraxians. (COMIC: In With the Tide)

The Third Doctor used Venusian Aikido to disable the hypnotised Ned. (COMIC: The Celluloid Midas)

The Fifth Doctor used Venusian Karate against Adric. (PROSE: First Born)

In an attempt to trick Curatrix Zennox and stop her from completing her plan to bring back the Cybermen, the Sixth Doctor claimed that he used Venusian Karate to overpower Jamie McCrimmon. She thought he was making that up. (AUDIO: Last of the Cybermen)

The Eighth Doctor, despite his amnesia, remembered a Venusian Aikido lesson and used it against Mo. (PROSE: The Eight Doctors) The Eighth Doctor used Venusian aikido against C'rizz to stop him from strangling Charley, although the Doctor later admitted that he was out of practice. (AUDIO: Faith Stealer)

Furthermore, the Twelfth Doctor drew upon his predecessor's practice of Venusian Aikido when confronted by the Sheriff of Nottingham's robot knights. (TV: Robot of Sherwood)

The Twelfth Doctor used Venusian aikido on Abesse after she repeatedly attacked him in the Prison canteen. He stated that he didn't use it on ladies as a general rule, but that Abesse wasn't a lady. (PROSE: The Blood Cell)

Others
Sarah Jane Smith knew Venusian Karate. (TV: A Girl's Best Friend)

River Song learned Venusian martial arts from the Venusian Martial Arts Academy on one of her honeymoons because she misread the brochure. (AUDIO: The Sonomancer)

Behind the scenes

 * Barry Letts was opposed to the idea of an aggressive Doctor. Therefore, Terrance Dicks suggested aikido as it has no attacking moves. (DOC: Can You Hear the Earth Scream?)
 * In the comic story Terrorformer, which is set after Robot of Sherwood, the Twelfth Doctor states that he has just remembered knowing Venusian karate, suggesting that Venusian karate and Venusian aikido may be different disciplines.