An outflow water clock, also known as a water thief or a water clock with outflow, is an ancient timekeeping device that relies on the steady flow of water to measure time. Other synonyms for outflow water clock include clepsydra, hydraulic clock, and water measure. The clepsydra, which means "water thief" in Greek, was used by ancient civilizations such as the Egyptians, Greeks, and Romans. Hydraulic clocks, on the other hand, were innovative devices used in medieval Europe and the Islamic world. The water measure was a type of outflow water clock that specifically measured the amount of water that flowed through it. These synonyms help us to understand the evolution and diversity of timekeeping methods throughout human history.