A clock is an instrument used for indicating and maintaining the time and passage thereof. The 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.