1. a level, as a surface of mercury, used in determining the altitudes of stars. Cf. level (def. 7).
2. the bubble in a sextant or octant for aerial use.
3. Also called flight indicator, gyro horizon. Aeron.an instrument that indicates the banking and pitch of an aircraft with respect to the horizon.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.