Kinktionary
More like guidelines and a lot less like definitions.
Face Fucking
Definition
Face fucking is a variant of oral sex where the person receiving oral stimulation uses a penis or other phallic object to actively penetrate the givers mouth
Similar to throat fucking.
General
The practice seems to originate in ancient Rome, where it was known as Irrumatio and used as a form of degrading punishment for persons of very low social status.
In a modern setting, this is practiced between two consenting partners, but the aspects of degradation and punishment may still apply in a playful manner.
Since the core act of face fucking consists of penetrating the givers mouth, it is often used to create a scenario where the receiver "uses" the givers mouth, which is surrendered freely.