Abatis

A type of man-made obstacle used in trenchworks or fortifications.  It consists of either a line of sharpened stakes, or cut tree limbs with sharpened branches and foliage oriented towards the enemy.

Abatis made with stakes (Courtesy of Pinterest)

Tree limb abatis (Courtesy of National Park Service

Casualty

Any loss in manpower that negatively effects a unit's fighting strength. This includes any soldier killed, wounded, missing or captured.

Enfilading Fire

The act of firing across the longest axis of an enemy unit or formation, allowing for maximum target density and reducing the number of enemy able to directly respond. It can be either flanking fire or frontal fire, depending on the enemy formation and direction of movement.

Examples of Enfilading Fire (Courtesy of Global Security)

Effects of enfilading fire on allied beach assault, WWII (Courtesy of Wikipedia)

Salient

A bulge in a defensive line.  While they can provide useful observation posts or jumping-off points for attacks, salients are comparatively isolated positions and are vulnerable to enemy assault from three sides.  Salients can vary in size drastically, but the same basic principles still apply.

Elliott's Salient, Petersburg (Courtesy of Stone Sentinels)

The Ypres Salient, World War I (Courtesy of NA Confidential)