An ambuscade is a surprise attack. There's nothing funnier than watching your cat lie in wait behind the couch until your dog wanders over, only to be startled by the cat's ambuscade.
While ambuscade is a bit old-fashioned, it's a perfectly good way to say ambush. Soldiers in battle conduct strategic ambuscades, and you may also enjoy using an ambuscade against your brother now and then, leaping out from behind a door to make him jump. It can also be used as a verb: "The dog is surprised every single time the cat ambuscades him."