hourglass

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An instrument for measuring time, consisting of two glass chambers connected by a narrow neck and containing a quantity of sand, mercury, or another flowing substance that trickles from the upper chamber to the lower in a fixed amount of time, often one hour.
2. adj. Shaped like an hourglass: an hourglass design; an hourglass figure.

Century Dictionary and Cyclopedia

1. n. An instrument for measuring time, consisting of a glass vessel constricted to a narrow passage in the middle, through which a quantity of sand, or sometimes of mercury, runs from the upper part into the lower in exactly an hour. At the end of the hour the glass may be reversed, when the sand will run back for another hour. Hour-glasses are now seldom used, though formerly very common. Similar instruments intended to mark shorter intervals are named accordingly, as a half-hour or a ten-minute glass. A three-minute glass, to boil eggs by. is called an egg-glass.
2. n. The time measured by an hour-glass; an hour.
3. Having the form of hour-glass.

Wiktionary

1. n. A clock made of two glass vessels connected with a narrow passage, with sand flowing through that passage.

GNU Webster's 1913

1. n. 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.

WordNet 3.0

1. n. a sandglass that runs for sixty minutes