When a good friend of yours becomes good friends with another good friend of yours.
Mike: My good friend Chris met my good friend Devlin today, and they liked each other!
Kathy: Worlds colliding!
When two separate aspects or relationships in a persons life collide: your friends (one "world") become friends with your girlfriend (a separate "world").
Popularized by The Seinfeld episode "the pool guy", in which Georges Costanzas' girlfriend forms her own friendship with Georges' platonic friend Elaine (Causing Georges' worlds to collide).
Kramer: 'Jerry, don't you see? This world here, this is George's sanctuary. If Susan comes into contact with this world, his world's collide. You know what happens then?'
<Kramer puts his hands together and makes explosion sounds that exemplify what happens when there are worlds colliding>
KRAMER: 'Ka shha shha shha Pkooo!'