noun: an event invloving two (only two) friends who sleep over at either friend's house.
verb: the act of two friends sleeping over.
A slumberover is restricted to 2 people. This is not to be confused with a "slumberparty" which involves 3 people or more.
Also slumberover is not to be confused with a sleepover. Sleepovers are not associated with fun. For instance, a person might sleepover alone at a hotel. That's no fun.
"Let's stay up late and gossip about people."
"You mean like a slumberparty?"
"No, stupid. Parties require 3 or more people. Since its just us two its gonna be a slumberover."