Keyblade Master is the title given to exceptional Keyblade wielders who pass the Mark of Mastery exam in the Kingdom Hearts series. There are a total of 13 known Keyblade Masters: Master Eraqus, Master Xehanort, Yen Sid, Aqua, Mickey Mouse, Riku, The Master of Masters, Invi, Ava, Gula, Ira, Aced, and Luxu. Currently, out of said 13 individuals, 9 remain active: Mickey, Aqua, Riku, Invi, Gula, Ira, Aced, Luxu and the Master of Masters.
Master Xehanort previously lost his memories, becoming the younger Terra-Xehanort, but was restored following the destruction of his Heartless and his Nobody. Yen Sid retired his title and passed his Keyblade down to Mickey, Eraqus was slain by Master Xehanort, who himself was slain by Sora.
A Master is not necessarily the most powerful Keyblade wielders as Master Xehanort, who himself was one of the most powerful Keyblade wielders alive, lost to both Sora and Terra. Sora has also defeated Aqua, yet another Keyblade Master, and Xehanort when he was possessing Terra's body. Like Mickey and Riku, Sora also has the Power of Waking, one of the most powerful abilities a Keyblade wielder or Keyblade Master can possess, despite not having the title of Master.
The Keyblade Master, upon passing the Mark of Mastery exam, is given the choice to either travel to many worlds and explore his or her horizons, or to succeed his or her predecessor and take refuge in his or her own world to protect it from malevolent forces and later take on more apprentices.
All Keyblade Masters share the ability to extract the hearts of themselves or others, this is shown when Master Xehanort extracts the hearts of Aurora and himself. They can also extract part of the heart as well, for instance, Xehanort extracts the darkness from Ventus's heart, and creates Vanitas. Luxu shows an even greated control of this ability, not even needing a keyblade to transfer his own heart into new vessels over the century.
Keyblade Masters also have the ability to perform the Keyblade Inheritance Ceremony, a trial in which a person is chosen to inherit the power of the Keyblade by clutching the Keyblade in their hand and allowing it to accept them. This is shown when Terra performs it on Riku, and when it is accidentally performed on Kairi when she grasps Aqua's Keyblade. Although, Terra does not have the title of Master, he is at the level of one, therefore he is able to perform this rite.
In addition to performing the Keyblade Inheritance Ceremony, Masters can conduct the Mark of Mastery exam upon potential Keyblade Masters. Although Yen Sid is a retired Master, he is still able to conduct the exam.
Also, Aqua is able to sense that the ceremony has been performed on Riku, although it is not known whether all Keyblade wielders can sense this, or if it is a Master's ability.