Clock is a tool for measuring, saving and indicating time. The word clock is derived from the Celtic languages clagan and clocca (through Dutch, Nordic French and Medieval Latin), meaning "bell". Traditionally, a static musical instrument without such a striking mechanism is called a clock. Today, in normal use, "clock" refers to any device used to measure and display time. Watches and other clocks that can be carried with you are usually different from clocks.

Clock is one of the oldest inventions of human beings, which can meet the needs of measuring the time interval shorter than the natural unit all the time: day, lunar month and year. Devices that run in several physical processes have been in use for thousands of years. Day D shows the time by showing the location of the shadow on the plane. There are many duration timers. A famous example is the hourglass. Water clock and day Di are probably the oldest timing tools. Great progress has been made in the invention of edge escapement device, which makes the earliest 1300 year mechanical clock in Europe possible. It keeps the cycloid and other cycloid for a long time. Spring driven clocks appeared in the 15th century. In the 15th and 16th centuries, the cause of clocks and watches flourished. With the invention of the pendulum clock, the next development in accuracy came after 1656. The main power to improve the accuracy and reliability of the clock is the importance of accurate timing for navigation. The electronic clock was patented in 1840. The development of electronic technology in the 20th century led to clocks without clock parts.

The timing element in each modern clock is a harmonic oscillator, that is, a physical object (resonator) that vibrates or oscillates at a specific frequency. This object can be a pendulum, tuning fork, quartz crystal, or the vibration of electrons in the atom when the atom emits microwaves. Analog clocks usually use angles to indicate time. The digital clock shows the digital representation of time. The digital clock usually uses two digital display formats: 24-hour system and 12-hour system. Most digital clocks use electronic mechanisms and LCD, LED or VFD displays. For convenience, distance, telephone or blindness, the auditory clock displays time as sound. There is also a clock for louvers, which has a display that can be read by touch. Some of them are similar to ordinary analog displays, but they are structured so that the touch does not damage them. The development of clock technology continues to today. The study of timing is called clocking.