noun Any device or mechanism that enables time to be measured or regulated.
A sundial is a type of clock that measures time through the movement of the sun's shadow.
noun In particular, a mechanical or electronic device that indicates the time of day through hands or a numeric display, and sometimes also through chimes.
The clock on the mantlepiece says midnight, but the church clock says quarter past.
noun A time clock.
I can't go off to lunch yet: I'm still on the clock.
noun An electrical signal that synchronizes timing among digital circuits of semiconductor chips or modules.
noun A CPU clock cycle, or T-state.
noun A watch (timepiece).
noun (Should we delete this sense?) A common noun relating to an instrument that measures or keeps track of time.
A 12-hour clock system; an antique clock sale; Acme is a clock manufacturer.