There is no official definition, but it is described in various ways:
https://en.wikipedia.org/wiki/Free-to-play#Criticism : In some multiplayer free-to-play games, players who are willing to pay for special items or downloadable content may be able to gain a...
Is DFO pay to win?
https://en.wikipedia.org/wiki/Free-to-play#Criticism : In some multiplayer free-to-play games, players who are willing to pay for special items or downloadable content may be able to gain a...
Is DFO pay to win?