An Ambush is the tactic of engaging enemy forces with a surprise attack. This gives the attackers the advantage by giving the opportunity to eliminate priority targets before the victims can return fire.
Typically, an ambush is begun when one or more players lie in wait for their enemy to pass by, especially on maps with significant choke-points like Laguna Presa and Port Valdez, where one can easily get the jump on a foe. The player ensures the killing of targets of priority, usually based on their capacity to return fire. Alternatively, an Assault or Recon in Bad Company 2 or Support in Battlefield 3 with C4, or the Engineer in either game with anti-tank mines is able to set traps on said chokepoints, which can destroy any armor and kill infantry (C4 only) without the danger of revealing the player's location.
Ambushes can also be enacted upon individual players rather than groups. One such tactic is to lie in wait for a lone, enemy Recon player to arrive in a popular camping spot, where within the player setting the ambuscade will use the melee attack on them.