An exchange of semen during oral sex.
A guy ejaculates into his partner's mouth during oral sex, and then semen is passed back into the mouth of the guy who just ejaculated through kissing.
This can involve multiple partners, and the term can also be used when the guy who just came didn't know his partner was going to transfer his semen back to him.
Fred's girlfriend is giving him oral sex; she ejaculates and she gives him a big wet french kiss before she swallows. Fred's been snowballed!