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.
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)
It seems his exclaimed name was employed as a battle cry.
Ace cried it out while throwing herself into an interstitial vortex to the dimension of Morgaine. (TV: Battlefield)
The Doctor would occasionally talk about him as a lead in to shout "Geronimo!" (AUDIO: Storm Warning, PROSE: The Hopes and Fears of All the Years, COMIC: The Golden Ones) He also used the expression alone on occasion. (AUDIO: Dead London)
The Eleventh Doctor used the expression following regeneration when his TARDIS was plummeting to Earth. (TV: The End of Time) He frequently used the expression. (TV: The Eleventh Hour, The Beast Below, etc.)
Dudley Jackson yelled out "Geronimo!" as he entered the time bubble of Stockbridge, likely towards his death. (AUDIO: The Eternal Summer)