1.(Astron.) A small constellation north of Aquila; the Arrow.
2.(Arch.) The keystone of an arch. (R.)
3.(Geom.) 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. (Obs.)
4.(Anat.) The larger of the two otoliths, or ear bones, found in most fishes.
5.(Zool.) 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.
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