This is the  military ring man's definition of the term:

Ambush is the military term for a surprise attack from a concealed force. The ambush attacks an enemy unit while it is passing by. The main purpose of the ambush is to totally destroy the force which is being atttacked.

In order to effectively kill off as much of the enemy as possible an ambush must catch them while they are in a kill zone. A kill zone is an area which limits the enemy's escape route. If they can't get away quickly there is more time to wipe them out. Normally when a vehicle convoy is ambushed the attackers try to kill off the first and last vehicles in the convoy to block the middle vehicles from escape.

There are three basic shapes for the ambush. The linear ambush follows a straight line such as a road, or river. The L shaped ambush has attackers positioned on one side to better kill the enemy. In a V shaped ambush the enemy is channeled into the kill zone so that they will be more effectively destroyed. Sometimes the enemy force can be lured into the kill zone and at other times the ambush point is determined by terrain features. You want to attack the enemy in a place where the natural land features makes it hard for them to escape such as next to a river, cliff, etc.

Since the ambush is normally behind enemy lines the ambushing force must be ready to get out of the area quickly.

The ambush begins with a predetermined signal and ends with another predetermined signal. There is normally an observation post placed apart from the ambush which notifies the attackers when the enemy is approaching.