1. (Science: astronomy) A small constellation north of aquila; the Arrow.
2. The keystone of an arch.
3. (Science: 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.
4. (Science: anatomy) The larger of the two otoliths, or ear bones, found in most fishes.
5. (Science: zoology) A genus of transparent, free-swimming marine worms having lateral and caudal fins, and capable of swimming rapidly. It is the type of the class Chaetognatha.
Origin: L, an arrow.