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My definition of Pay2Win is a game with a Cash Shop where the shop sells items that actually give you an advantage over someone else, especially in PvP situations. It does not include XP boosters or anything like that especially if those things are found in game. Basically, it's armor, or weapons (or anything else) that are overpowered compared to what you can find in the game itself, especially if those things can be used against other players.
Areas in the game that are necessary for progression but are locked unless you pay for them are also (by definition) Pay2Win.
I would even go so far as to say that it could be gear that allows you to, say get through a 5 man dungeon by yourself, if you can't also get that gear in game. Or if it's extremely difficult to get that gear in the game.
There has been a lot of talk lately about Pay2Win Cash Shops in games, but I believe most of that talk is hyperbole and/or misinformed. What do you think?
Edit: Added the comment about areas in the game.