An ambush is a sneak attack. To ambush your enemy, hide and wait for him to come near and then pounce on him.
In war or in backyards, an ambush is a great way to surprise someone. Ambush comes from a Latin word meaning “to place in a wood,” and hiding in the woods behind a tree is a classic starting point for an ambush. The actual attack is called an ambush, but ambush is also a verb, so you can ambush your mom by dropping water balloons on her head from a tree. Please don’t tell her where you got that idea.