An uncontrollable burst of laughter
Usually done in an otherwise unfunny situation where something unexpected has occured that has made a person laugh, believing the unexpected thing to be a joke.
This can make the situation incredibly awkward once the person stops laughing.
Landa: Mountain climbing? That's how you injured your leg- mountain climbing?
Hammersmark: Believe it or not, yes it is.
Landa: *Bursts out laughing*
Janet: I'm seeing someone else.
Janet: Graham Jenkins....
Todd: *Bursts out laughing. Pulls himself together*
Seriously though, who are you seeing?