Enfilade and defilade are concepts in military tactics used to describe a military formation's exposure to enemy fire. In addition, enfilade fire is used to describe gunfire directed against an "enfiladed" formation or position. The words themselves come from French (enfiler - to skewer; défiler - to scroll). This concept is not only important in firefights, but can also be demonstrated in many first person shooter video games.
In the broadest sense, these terms simply mean an advantageous position to fire from (enfilade) or a position of cover from incoming fire (defilade). Technically, a formation or position is "in enfilade" (or enfilading) if weapons fire can be directed along its longest axis. For instance, if a team is able to fire upon an enemy trench down the length of the trench, then they are enfiladed. A column of marching troops is enfiladed if fired on from the front or rear such that the projectiles travel (and threaten the enemy) the length of the column.
The benefit of enfilading an enemy formation is that, by firing along the long axis, it is easier to hit individual troops within that formation. Adjusting the elevation of the weapon merely directs the fire to a different point along the axis of the formation, although traversing the weapon is more likely to result in a miss. Enfilade fire takes advantage of the fact that aiming at a target is easier than correctly estimating the range to avoid shooting too long or short. Finally, projectiles that miss an intended target are more likely to hit a different target within the formation if firing along the long axis.
Strafing, firing on targets from a flying platform, is often done by enfilade fire when using forward weapons, and defilade fire when using side-mounted weapons. Against zombies, firing high caliber rounds down a narrow hallway of infected is an excellent example of enfilade fire. They are difficult to miss, and each round has a chance at penetrating multiple zombies.
Technically, a unit or position is "in defilade" (or defilading) if it uses natural or artificial obstacles to shield or conceal - particularly when referring to a position on the reverse slope of a hill or within a depression in level or rolling terrain. Defiladed positions on hilltops are advantageous because they allow a defender to take advantage of the height of the terrain without suffering the disadvantage of being silhouetted against the sky. However, because of the slope, "dead space" that cannot be engaged with direct fire will be created in front of the position.
More practical examples of defilades are windows, trenches, sandbag walls, and to a limited extent, cars and other improvised cover.