Professional smasher
A professional gamer is one who regularly makes money off of the game and/or one who is recruited and sponsored by a notable organization, such as MLG. Due to MLG and other major gaming corporations not sponsoring Smash Bros. players anymore, a professional is usually considered the person at the top of the metagame of that said character, such as Hax for Captain Falcon or Axe for Pikachu.
"Professional" may also refer to the player's skill level, where a player has reached the proficiency threshold necessary to be able to consistently rank high in tournaments where he/she is pinned against other players deemed "professionals".
Semi-professional
A semi-professional, or a semi-pro, is a player with above-average skill who lacks the consistency or overall prowess to consistently rank high in tournaments.
Top professional
A top professional is a player who is among the very best from his country and/or for their respective main character. Notable examples include Salem and Mew2King for Brawl, Mango and Armada for Melee, Isai and SuperBoomFan for Smash 64.
