A digital clock is a type of clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 that displays the time digital

Digital

A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...

ly, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Digital clocks are often associated with electronic drives, but the "digital" description refers only to the display, not to the drive mechanism. (Both analog and digital clocks can be driven either mechanically or electronically, but "clockwork" mechanisms with digital displays are rare.) The biggest digital clock is the Lichtzeitpegel ("Light Time Level") on the television tower Rheinturm Düsseldorf

Rheinturm Düsseldorf

The Rheinturm is a 240.5 metre high concrete telecommunications tower in Düsseldorf, capital of the federal state of North Rhine-Westphalia, Germany. Construction commenced in 1979 and finished in 1981. The Rheinturm carries aerials for directional radio, FM and TV transmitters...

, Germany.
Perhaps the earliest digital timepieces were Plato clocks.