Kinktionary
More like guidelines and a lot less like definitions.
Pay For Play
Pay for play indicates that a person requires money in exchange for sexual favors. The person requiring payment may be a professional sex worker who caters to a specific kink or BDSM practice. Pay for play is not limited specifically to professional sex workers and may be found in any type of sexual, kink, or BDSM practice.
In general, these commercial transactions can be for any kind of play and can be acquired in the s-type and D-type variants or any other preferences. In the transaction, the sex worker is often specialized and has a background in kink/BDSM themselves. As it is more of a niche market, these professionals are only a small subsection of the larger group of sexworkers.
Note that not all pay for play happens with professionals. In some cases, people with a preference for FinDom/FinDomme also do these things. Also some people with a kink for being payed for their sexual or kink favor can do these activities.