Enfilade and defilade are military tactical concepts used to describe a fighting unit's exposure to enemy fire. In addition, enfilade fire is used to describe gunfire directed against an "enfiladed" unit or position.
A unit (or position) is "in enfilade" if enemy fire can be directed along the long axis of the unit. For instance, a trench is enfiladed if the enemy can fire down the length of the trench. A column of marching troops are enfiladed if fired on from the front such that the gunfire traverses the length of the column. A line of advancing troops is enfiladed if fired on from the flank.
The benefit of enfilading the enemy is that, by firing along the long axis, it is relatively easy to hit a target (or several targets). Raising or lowering the aim of a weapon merely directs the fire to a different point along the axis of the position. The less desirable alternative is to fire from an oblique angle which requires precise aim to hit a target. Raising or lowering the aim when firing obliquely results in the target being straddled.
A unit or position is "defiladed" if it is protected from direct exposure to enemy fire. For an armoured fighting vehicle, defilade can be synonymous with hull-down.