he word "clock" is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". For horologists and other specialists the term "clock" continues to mean exclusively a device with a striking mechanism for announcing intervals of time acoustically, by ringing a bell, a set of chimes, or a gong. A silent instrument lacking such a mechanism has traditionally been known as a timepiece. In general usage today, however, a "clock" refers to any device for measuring and displaying the time which, unlike a watch, is not worn on the person.

The clock is one of the oldest human inventions, meeting the need to consistently measured intervals of time shorter than the natural units, the day, the lunar month, and the year. Such measurement requires devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today. In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water clock.

None of the first clocks survived from 13th century in Europe, but various mentions in church records reveal some of the early history of the clock. Medieval religious institutions required clocks to measure and indicate the passing of time because, for many centuries, daily prayer and work schedules had to be strictly regulated. This was done by various types of time-telling and recording devices, such as water clocks, sundials and marked candles, probably used in a combination. Important times and durations were broadcast by bells, rung either by hand or by some mechanical device such as a falling weight or rotating beater.

The word horologia (from the Greek, hour, and to tell) was used to describe all these devices, but the use of this word (still used in several romance languages) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a ‘horologe’ but the mechanism used is unknown. According to Jocelin of Brakelond, in 1198 during a fire at the abbey of St Edmundsbury (now Bury St Edmunds), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire. These early clocks may not have used hands or dials, but “told” the time with audible signals