1. (n.) A small constellation north of Aquila; the Arrow.

2. (n.) The keystone of an arch.

3. (n.) 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. (n.) The larger of the two otoliths, or ear bones, found in most fishes.

5. (n.) 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.