What is a "Reentrant"?

Apart from a few geologists, only orienteers regularly use the English word "re-entrant" (or reentrant) to describe a landform.

A reentrant appears on the map as a U or V shape in the contour lines, pointing back into a hillside rather than sticking out of the hill (as would a spur). So a reentrant is a small valley, the center of which would collect water and funnel it downhill (if it were raining hard). This portion of a map includes several reentrants, three of which are circled. The west-most is a small, v-shaped reentrant, while the two eastern examples are broad and somewhat shallow.