The natural tendancy for a group of people (consisting of more than three) to, at some point during a gathering, become amassed on a bed or bed-like area (a couch, the backseat of a car, possibly even the floor) in a slightly compromising manner.
This is not to be mistaken for an orgy, or anything of the sort. Clothes are (generally) left entirely in place. The only defining characteristic is people lying together.
It has been postulated that this effect is caused by an unknown element residing within our universe. Accepting that any new-found element would be most likely radioactive, the as of yet undiscovered element is thought to contain only a partial proton in its nucleus giving it a fractional atomic number.
Last night, due to the Kaiser effect, we broke my box spring.