1. A small constellation north of Aquila; the Arrow.
  2. The keystone of an arch.
  3. 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. The larger of the two otoliths, or ear bones, found in most fishes.
  5. 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.