AFD for short. This is when a person becomes an uber fangirl/fanboy of whatever they enjoy to the point where they are fans of everything.
Person 1: Omg! I love Harry Potter!
Person 2: OMG YEAH! And I love Twilight and Star Trek and Lord of the Rings and Pokemon and sheep and chapstick and ipods and cheese and...
Person 1: You so have Addictive Fandom Disorder.
Long: An effect in which two substances or actions used in combination produce a total effect the same as the sum of the individual effects.
Short: 2 actions combine to make one big one
Gary: "Yo, wanna combine our soda together to make an additive effect?"
Charlie: "Sure! You put your Sprite first and then I will do mine"
Gary: "We have made such a large sprite"
Charlie: "Indeed"