In tennis (and for golf too, I believe), players are said to turn pro after a few years of Challenger and lower-grade Tennis. What does the phrase mean?
It's a bit of a nebulous term, really. I am not familiar with the world of golf enough to speak on the details of "turning pro" in that sport, but in tennis it really just boils down to two things:
- You have requested and been given an IPIN number by the ITF
- You are playing in an ITF, ATP or WTA event (Futures, Challengers or Tour level event) and you are accepting any prize money you might earn from your participation in the event.
Some younger players (teenagers, typically) that are good enough to qualify for pro level events that pay prize money have to make a decision about whether to accept the money or not in order to maintain their "amateur" status in the sport since playing college tennis usually means you cannot have been considered a "pro" before.
It depends on the sport; some have very strictly defined rules for "amateur" versus "professional". Golf is one of them, and I believe tennis also makes this distinction.
Basically, if you've ever been paid to play the game, or have accepted a prize for winning an event beyond a certain monetary value, you are said to have lost your "amateur status"; you are now a "professional" player, and cannot compete in events that are open to amateurs only, under threat of disqualification from the event, as well as possible criminal charges if by fraudulently competing as an amateur you win money or a prize.
There are some exceptions; there is a minimum prize value that triggers this change, so amateur tournaments can give meaningful, valuable prizes to players without causing those players to unwittingly "turn pro". Additionally, token payments for instruction, like "gas money" or buying your instructor a beer in the clubhouse, don't trigger loss of amateur status.
Additionally, an amateur player can, without any money changing hands, "renounce" his or her amateur status at any time. This is rarely done; typically a player will announce they are "turning pro" by accepting something that would invalidate their amateur status as a matter of course, such as taking the top prize at the U.S. Open or accepting a sponsorship deal.
In most other circumstances, being a "professional" simply means you're paid to play the game. Some make a distinction between "semi-pro" (yeah, you're paid to do it but you still need a "day job") and truly "professional" (you make enough that playing is your day job). A few sports will allow players who have previously played professionally to compete at lower levels, ranking players more on current skill level (so a 60-year-old former NHL player might be welcomed into a B or C-league hockey team), but others do still make the distinction that if you've ever played professionally, you can't go back.