The Collaborative International Dictionary
Ravelin \Rave"lin\ (r[a^]v"l[i^]n; 277), n. [F.; cf. Sp. rebellin, It. revellino, rivellino; perhaps fr. L. re- again + vallum wall.] (Fort.) A detached work with two embankments which make a salient angle. It is raised before the curtain on the counterscarp of the place. Formerly called demilune and half-moon.
Wiktionary
a. (context of furniture English) In the shape of a half-moon, i.e. semicircular. n. 1 A fortification constructed beyond the main ditch of a fortress, and in front of the curtain between two bastions, intended to defend the curtain; a ravelin. 2 (context biology English) A crescentic mass of granular protoplasm present in the salivary glands.
Usage examples of "demilune".
It was a half-moon of stone or concrete, a demilune, a strongpoint built out from a narrow ledge of the cliff.
Thirty feet below the demilune, the corpse of the second German swung like a clock pendulum over the dizzy swoop of the cliff and the rock-smashed waves at its base.
Other than the two demilunes outside, the four corner bastions, and the ramparts themselves, that's the setup aboveground.
They had enclosed this peninsula in a modern sort of slope-sided wall, a couple of miles in circuit, and erected noble bulwarks and demilunes at its corners, rendering it impregnable to land assault by Dutch, Chinese, or native legions.
There are some weak points, but I have reinforced them with demilunes and batteries.