an instrument for measuring time by the trickling of sand, mercury, water, etc. through a small opening from one glass bulb to another below it, in a fixed period of time, esp. one hour
See hourglass in American Heritage Dictionary 4
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.
Shaped like an hourglass: an hourglass design; an hourglass figure.