The narrow part in the middle of a standard hourglass is called the waist. This constricted opening connects the two glass bulbs and controls the flow of sand from the upper chamber to the lower one.
An hourglass measures a fixed interval by allowing a granular material—usually sand or another carefully selected substance—to pass through a small opening under gravity. The waist is shaped and sized to produce a reasonably consistent flow, but the device is not a precision clock. Humidity, grain size, static electricity, and the hourglass’s orientation can all affect the result.
Hourglasses appeared in Europe by the 14th century and became useful for timing sermons, cooking, navigation, work periods, and games. Sailors used larger maritime sandglasses to help estimate time at sea, often in half-hour intervals.
The waist is sometimes informally called the neck because it resembles the narrow neck of a bottle. “Waist” is the standard descriptive term for the central constriction, while “hourglass” can refer to timers lasting many intervals—not only one hour.