The act of sex/intercourse but being cut off before you can finish for other reasons unknown.
Taking a girl home, proceeding to bang her but then when your maybe half way through, she decides to quit cos shes had enough fun.
"Cory takes Morgan home to finger her with his penis, Morgan decides shes had enough fun and tells him, "I'm done". Cory is left there with his unfinished business. This explains fraction sex.