Geronimo

Geronimo was a Native American friend of the Doctor's, also known as Goyaale (the One Who Yawns). (PROSE: The Hopes and Fears of All the Years) The Doctor knew him as early as his eighth incarnation. (PROSE: The Shadows of Avalon; AUDIO: Storm Warning)

He was the leader of the Apache tribe. (PROSE: Avatars of the Intelligence)

It occurred to the Eighth Doctor that Geronimo would have been puzzled to know he would be remembered mainly by people who were about to fall from a great height. (PROSE: The Shadows of Avalon) The Eighth Doctor joked to Charlotte Pollard that Geronimo taught him the greeting "". (AUDIO: Storm Warning)

Name used as an exclamation
Geronimo's name later came to be popularised as a battle cry, often uttered before someone attempted to do something daring or dangerous. The Fourth Doctor, (AUDIO: White Ghosts) Sixth Doctor, (AUDIO: The Crimes of Thomas Brewster) Eighth Doctor, (AUDIO: Dead London) Ninth Doctor (COMIC: The Love Invasion) and Tenth Doctor (PROSE: The Hopes and Fears of All the Years) all used the word.

The Eleventh Doctor adopted the word as a catchphrase, often shouting it out before engaging in a dangerous activity. (TV: The End of Time, The Eleventh Hour, The Beast Below, et al.) The Twelfth Doctor used the phrase once, alongside Clara Oswald, as a signal to call in the Paternoster Gang. (TV: Deep Breath) The Thirteenth Doctor briefly pondered what it was she used to yell before leaping into the unknown as she made her way down a secret passageway, not remembering what it was until she had already landed at the bottom. (COMIC: A New Beginning) She later used it when facing off against Sea Devils. (TV: Legend of the Sea Devils)

Companions and others
Ace cried it out while throwing herself into an interstitial vortex to the dimension of Morgaine. (TV: Battlefield) Bernice Summerfield used the expression on at least one occasion. (AUDIO: Oh No It Isn't!) Maxwell Edison yelled out "Geronimo!" as he entered the time bubble of Stockbridge, likely towards his death. (AUDIO: The Eternal Summer) When the Eleventh Doctor, thinking that Amy Pond was scared, tried to comfort her, she yelled "Geronimo" to show him that she wasn't scared. (TV: Asylum of the Daleks) Clara Oswald yelled "Geronimo" when she was forced to jump out of a spaceship passing through a wormhole, in hopes of the Eleventh Doctor catching her with his TARDIS. (COMIC: Sky Jacks) Henry Gordon Jago once used the expression. (AUDIO: The Necropolis Express)

Behind the scenes
Geronimo appears as a card in the 1978 Doctor Who Trump Card Game by Jotastar.