noun

• The keystone of an arch.
• (geometry) The distance from a point in a curve to the chord; also, the versed sine of an arc; so called from its resemblance to an arrow resting on the bow and string.
• (zootomy) The larger of the two otoliths, or earbones, found in most fishes.
• Any arrowworm, of the genus Sagitta.
• (astronomy) A small autumn constellation of the northern sky, said to resemble an arrow. It lies between the constellations of Vulpecula and Aquila.