When you're watching a japanese porno and the guy finishes inside her pussy. Usually the guy inserts his grubby fingers into her pussy and tries to scoop out the non-existing cum
Mike: what's your favorite fetish?
John: I love japanese creampies
Mike: dude, that's nasty
The act of unprotected/uncovered penile ejaculation and insemination inside the vagina.
E.g For the a couple attempting conception/procreation the male will release his sperm (the 'cream') into the female's vagina during climax/orgasm thus releasing his cream into the 'pie.' When withdrawing his penis typically some of his seed will drool/ooze out of the vagina, giving the creampie after-look.
Female - 'He came so much inside me it felt like it was drooling out forever... The perfect creampie!'