Pokémon Master
It is the goal of Ash Ketchum, as well as many other Pokémon trainers, to become a Pokémon Master, however - exactly what this position is and how one attains it is vauge and never fully explained.
It has been theorized that when one becomes a Pokémon Champion of a regional Pokémon League, one attains the title of master - however, this cannot be correct, as Ash won the Orange League without attaining the position. Indeed, other League Champions haven't ever been referred to as Pokémon Masters, either.
Another popular theory is that one must catch one of every kind of Pokémon in the world to become a Pokémon Master, however this is likely to be incorrect as far as the Pokémon anime is concerned as well. For one, it would be impractial, as capturing Legendary Pokémon is both extremely difficult, and is often shown to be morally wrong. In addition, various negative and dangerous effects appear to come as a result of capturing Legendary Pokémon.
Aside from the Legendary issue, it would be impractical for anyone in the anime to capture and maintain 391 different creatures. Unlike the Pokémon games or Pokémon Special manga, in which Pokémon can be stored in suspended animation as computer data - Pokémon in the anime need t be fed and exercised regularly to maintain their health. Such a task would be impossible for anyone to perform alone, or even with many assistants or helpers.
So what defines a Pokémon Master? The answer may lie by looking at a real-life skill that also has trouble defining a master - martial arts. In that field, as in the world of Pokémon training, even winning tournaments, belts, and competitions doesn't define if someone has achieved the status of a "master". Instead, masters are defined more by inner character, inner strength, and intuition, rather than who has the most power or skills.
Indeed, a Pokémon Master may not be someone who has won a title, so much as it may simply be someone who has strived so dilligently, that Pokémon training and befriending Pokémon is intuitive.
According to the Electric Tale of Pikachu manga, a Pokémon Master is simply an elite trainer who is considered a professional and regularly takes part in League competitions.
