sagitta

noun4

1. •
   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.
2. •
   Any arrowworm, of the genus Sagitta.
3. •
   The larger of the two otoliths, or earbones, found in most fishes.
4. •
   The keystone of an arch.