What does "hourglass" mean?

Hourglass: An instrument for measuring time, especially the interval of an hour. It consists of a glass vessel having two compartments, from the uppermost of which a quantity of sand, water, or mercury occupies an hour in running through a small aperture unto the lower. Note: A similar instrument measuring any other interval of time takes its name from the interval measured; as, a half-hour glass, a half- minute glass. A three-minute glass is sometimes called an egg-glass, from being used to time the boiling of eggs.