A military crest sounds like it belongs on top of a 19th-century officer’s hat or helmet, but the term refers to a terrain feature:
Military crest is a term in military science that refers to, “An area on the forward or reverse slope of a hill or ridge just below the topographical crest from which maximum observation and direct fire covering the slope down to the base of the hill or ridge can be obtained.”
The military crest is used in maneuvering along the side of a hill or ridge to provide the maneuvering force maximum visibility of the terrain below and minimize their own visibility by not being silhouetted against the sky, as it would be at the topographical crest of the hill.
Observation points (OPs) can be located at the military crest if the main defensive position is located on the reverse slope of the hill or ridge, as is usually done if the main defensive position would be vulnerable to the enemy’s artillery if located at the military crest, making coordinated withdrawal difficult.
(Scipio Americanus used the term recently in discussing the Defence of Duffer’s Drift.)