Imo,
Main: a player who mainly plays a specific hero, or a number of heroes. For example, a player could be a Lucio main, whilst another player could be a Mei and Orisa main. Or, a player may main a role, as such as the Support role. These players can still switch off to another hero in the time of need.
One trick: a player who only plays 1 hero, all the time, and they do not switch off. For example, a player could be a Dva one trick and will only ever play Dva.
Also ill mention Flex, because why not. A flex player is flexible in their hero choices and can play a wide variety of heroes to a good standard (if not all heroes, but idk).