1. (noun) Sex with someone so easy (or ugly) that they don't even count as a full notch on your belt. Similar to Fictional Sex, sex that's either made up to boost one's numbers, or falsely denied to reduce the bed count.
2. Sex that's so lousy, it doesn't even count as a full fuck.
3. Sex that's so amazing, or so difficult to score, it counts as several notches on your belt.
1. Bob: "Man, I fucked 1/3 of a girl last night."
Dan: "What do you mean? Where was the rest of her?"
Bob: "It was fractional sex, this chick named Amy. All I had to do was buy her one drink and she was back at my place. She was so easy, it doesn't even count as a full lay."
2. Krista: "How many guys have you been with, Amy?"
Amy: "Two and a quarter?"
Krista: "Two and a quarter? How do you fuck 25% of a guy?"
Amy: "It was this guy name Dan. He was so shitty in bed, it don't even count as a full lay. He had a little dick and he only lasted two minutes."
Krista: "Ah, fractional sex. I've had a few of those. I'm up to 3.4 I think."
Amy: "3.4, huh? Sounds like fictional sex to me. You're forgetting about those two guys you did on Spring Break, and that Jerry guy, and that professor you went down on, and-"
Krista: "Shut up! You made your point."
3. Will: "Well, it took me seven months, $500 in dating expenses, and four bullshit love poems, but I finally got Destiny in bed last night. And man, it was so worth it! She blew my fucking mind! I swear that counts as ten notches on my belt right there."
Steve: "Ah yeah, fractional action, huh? Nice."