Which of these is a term for 'clepsydra'?

The term "clepsydra" specifically refers to a water clock, which is an ancient time-measuring device that uses the flow of water to measure time intervals. This type of clock typically consists of a container that fills with or drains water at a consistent rate, allowing for the measurement of time based on the amount of water used. The name itself is derived from Greek roots meaning "water" (kleptein) and "to steal" (hydra).

Other options refer to different types of time-measuring devices: a sand timer (often called an hourglass) uses sand to track time; an hourglass is a specific kind of sand timer; and a sun clock refers to sundials that use the position of the sun to measure time. Since these devices operate on different principles and materials (sand or sunlight), they are distinctly different from a clepsydra, which relies solely on water.