Word "professional" does not come from word "expert" or "master", "average" or "buddy" it comes from "profession". One can easily conclude that "Professional" poker player therefore is a player who earns living from playing poker. If you can make it, no matter how good/average you are - you are a professional poker player.
I think it comes down entirely to percentage income. If you're earning around 80% income from poker and above, you can call yourself professional. Above around 30% and you're semi-pro and anything less and it's a hobby that makes a bit of money on the side