The power to master every aspect of an ability to the fullest extent.
The user has mastered the ability in question, not just being proficient in its usage, but also understood every concept and possibility available. Those who mastered the one ability may be able to even create concepts of such power that normal users are not capable of, as well as become resistant to having their power negated, among many other possibilities.
Soul Reapers who mastered their Zanpakuto (Bleach)
Dignified Power Users (Code: Breaker)
Adelaide Grand Marnier (Dog Days)
Albus Dumbledore (Harry Potter)
Lord Voldemort (Harry Potter)
Shichirou Ougi (Kekkaishi)
Hashirama Senju (Naruto)
Hiruzen Sarutobi (Naruto)
Itachi Uchiha (Naruto)
Madara Uchiha (Naruto)
Tobirama Senju (Naruto)
Gemma Himuro (Ninja Scroll)
Monkey D. Luffy (One Piece)
Jabra (One Piece)
Life Return users (One Piece)
Logia Devil Fruit users (One Piece)
Ranma Saotome (Ranma 1/2)
Akua Shuzen (Rosario + Vampire II)
Goku (Dragon Ball)
Gohan (Dragon Ball)
Vegeta (Dragon Ball)
Frieza (Dragon Ball)
Medaka Kurokami (Medaka Box)
Heaven Ascension DIO (Jojo's Bizarre Adventure: Eyes of Heaven)
Mei Mei (Valkyrie Crusade)
Mikoto Misaka (A Certain Magical Index) has mastered her electricity manipulation skills to the point of hacking, electromagnetic wave reading, and many other versatile applications.
After mastering Eagle Vision, Ezio Auditore da Firenze (Assassin's Creed) gained Eagle Sense, enabling him to sense where targets have been or where they're going, pick up cold trails, and see through enemy disguises.
Yamamoto (Bleach) has mastered the many arts of a Soul Reaper, the most prominent is his swordsmanship, and is considered to be the strongest Soul Reaper within the last 1000 years.
Sylar (Heroes) has mastered the use of Telekinesis, allowing him to do such things as flip trucks, precisely cut his victims, and augment his own strength and durability.
Madara Uchiha (Naruto) has mastered his eyes in a very young age, along with a plethora of abilities such as Fire Release, Wood Release, hand-to-hand and sword combat.
Gemma Himuro (Ninja Scroll) has mastered the control of his entire body down to the tiniest bone and blood, a feat unachievable by human standards, and has acquired immortality.
Monkey D. Luffy (One Piece) mastered his rubber body physiology to the point of stretching his blood vessels to increase oxygen transfer and pump air to harden his bones.
Akua Shuzen (Rosario + Vampire II) has mastered Jigen-Tou ability to the point of extending the intangibility to her entire body for defense, as well as the range of her offensive might.
Goku and Gohan (Dragon Ball) have mastered the base Super Saiyan form to the extent that they can remain in it for days with no ill effect, even while sleeping.
After mastering the power of the Super Saiyan God form, Goku and Vegeta (Dragon Ball) were able to apply godly Ki to their Super Saiyan forms, achieving the form of Super Saiyan Blue.
After extensive training in preparation for his rematch with Goku, Frieza (Dragon Ball) was able to fully master his full power, allowing him to use it without bulking up his muscles and thus negating the stamina-draining drawbacks.
Vegeta (Dragon Ball) has mastered the Great Ape form, allowing him to retain his intelligence and speech capabilities, as well as his speed and power to fly.
Mei Mei (Valkyrie Crusade) can copy and master any ability/technique she sees.
Megumin (Konosuba) has mastered the Explosion spell to the absolute exclusion of all other magics, granting her power beyond that of even other Crimson Demon Arch-Wizards.