So far we have seen occupation, career, mindset, natural predisposition, even genetic disposition.
Mindset, perhaps, but I do not agree with any of the other descriptions of a RM profession.
To me, professions are not an attempt to be realistic about careers or predispositions.
Professions are first and foremost foundations of and catalysts for a great fantasy setting.
Professions bring archetypes into the game, and serve as inspiration for the players.
Hence a profession can be a chosen path, an idea, a hope of excellence, or the notion of fulfillment of purpose.
I would not allow a player to change his character's profession, even if the character's chosen path could realistically be strayed from.
I feel that for the sake of the game, the player should be committed to his choice.