After countless times listening to the swelling exposition of Avatar: The Last Airbender, any fan of the series knows that only the Avatar can master all four elements. Aang's journey to master the elements served as the bulk of his overarching journey throughout the original series, and the plots of The Legend of Korra and the Kyoshi novels centered on much the same process. That word "master" gets thrown around quite a lot in the Avatar series, yet many fans seem to overlook an obvious problem with the usage of the term: Nobody really seems to know what it means.
Aang's initial arc seems to set him up to need to master all four elements in order to defeat the Fire Lord. Yet by the series end, he seems to fall short of that goal. Even going into the finale he says that he needs to continue practicing firebending, while Toph tells him that his earthbending also needs work. Though the audience already knew that Aang was a master airbender, what proves interesting about the exchange is the implication that he had already mastered waterbending. At first, that would seem to make sense given that waterbending came naturally to Aang and he had far more time to practice and master the element compared to earth and fire.
What proves puzzling, however, is an actual comparison of Aang's waterbending to his earthbending. It becomes rather hard to identify just what exactly it is about his usage of one that seems so much more skilled than the other. Aang performed basic attacks and defenses with both elements with seemingly equal proficiency, and if anything, his showings creating massive ripples of earth during the invasion of the Earth King's palace and the battle in the Crystal Catacombs were considerably more impressive than anything he did with water.
This begs the question: at what point, exactly, does a bender becomes a master? The term gets thrown around in most catalogs of Avatar characters with great abandon, regardless of the abilities the Bender displays. The Firebender Zhao, the Waterbender Tonraq and the Earthbender Yu are all referred to as "masters" of their respective elements despite all three showing abysmal records in their duels against others. Beyond unnamed fodder, they never showcase anything approaching unique or powerful in how they wield their elements.
Yu, in particular, bears the title "master" more often than most, and his reason for doing so may point to the fact that the honor of mastery is one that is not exactly hard to achieve. The author of the Kyoshi novels, F.C. Yee, briefly mentioned in an interview with CBR that mastery, in a sense, referred to a Bender's ability to teach their skill to others. Master Yu's for-profit school in Gaoling where he taught earthbending fundamentals to anyone with enough coin demonstrated that this was not a high bar. Yu himself only ever demonstrated moves most Earthbenders are capable of, and barely put up a fight against Toph -- who easily disposed of him.
Such fights like Yu and Toph's brief bout could well indicate exactly why the term "master" gets thrown around so often in the fandom: The more impressive lower-level fighters seem then the more impressive the higher-level fighters who beat them become. Such usage of the term robs "master" of any real meaning and instead supposes that the entirety of the Avatar series across TV, comics and prose consist of casts with over a dozen masters so masterful that they masterfully fight against other masters so proficiently that their foes seem amateurish by comparison. For the sake of preserving the believability and integrity of the world, such overusage of the term should stop.
Not everyone can be a top tier Bender demonstrating the very best of what their art form has to offer. Instead, use of the term "master" should be as ceremonial and formalistic as it is with the Airbenders -- the only one of the four bending arts with any qualifications required to earn the rank. Said qualifications could still use further illumination, but as is the standard of completing the 35-36 tiers of airbending and inventing a new technique, there's a far clearer demarcation between those who deserve the rank and those who do not. The consequence is a feeling of immense reward when a character actually earns it.
Subsequent installments in the franchise would do well to enlighten fans as to what the term truly means and which Benders truly deserve it. It may even prove a unique bit of worldbuilding to see what represents mastery in each of the elements, be it the integration of subskills like lightning-generation or metalbending, or a question of raw power and the amount of mass a Bender can manipulate at once. As is, the term is bandied about so much it's basically meaningless. If referring to a character as a master of their artform is to regain the level of honor it should carry, defining what exactly it means is practically a necessity.