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.
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.
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 1,000 years.
Sosuke Aizen (Bleach) has completely mastered Zanjutsu, Shunpo, Kido and Hakuda, making him one of the most powerful Soul Reapers in existance.
Yoruichi Shihoin (Bleach) has mastered Shunpo to the extend that she can defeat entire squadron of the Onmitsukido in seconds.
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. Lufy (One Piece) has mastered his Gomu Gomu no Mi powers to such an extent that he can stretch his blood vessels to increase oxygen transfer...
...pump air to harden and enlarge his bones...
...and combine Busoushoku/Armament Haki with his rubber body, allowing him to enter his Gear Fourth - Bounce Man form.
Donquixote Doflamingo (One Piece) has mastered his Ito Ito no Mi to the point where he can repair his internal organs, create a birdcage the size of an island, create a doppelganger of himself, control people up to an entire population, etc. by using strings.
Charlotte Katakuri (One Piece) has mastered his Kenbunshoku Haki to the point he can see the future.
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.
During his battle with Jiren, Goku (Dragon Ball) was able to completely master Ultra Instinct and assume his most powerful state of being, Instinct Complete.
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.
Through rigorous mental training while in Hell, Frieza (Dragon Ball) was able to master his Golden form, allowing him to use it without losing stamina.
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.
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.
Esdeath (Akame ga Kill) has mastered the ice based powers of her Teigu Demon's Extract to the point she freeze space-time and even create massive ice storms that can annhilate a continent.
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.
Mei Mei (Valkyrie Crusade) can copy and master any ability/technique she sees.