- a level, as a surface of mercury, used in determining the altitudes of stars.
- the bubble in a sextant or octant for aerial use.
- Also called flight indicator, . Aeronautics an instrument that indicates the banking and pitch of an aircraft with respect to the horizon.
- Also called: gyro horizon an aircraft instrument, using a gyroscope, that indicates the aircraft’s attitude in relation to the horizontal
- astronomy a level reflecting surface, such as one of mercury, that measures the altitude of a celestial body as half the angle between the body and its reflection