Time is the never-ending continued progress of existence and events. It happens in an apparently irreversable way from the past, through the present to the future.
To measure time, we can use anything that repeats itself regularly. One example is the start of a new day (as Earth rotates on its axis). Two more are the phases of the moon (as it orbits the Earth), and the seasons of the year (as the Earth orbits the Sun). Even in ancient times, people developed calendars to keep track of the number of days in a year. They also developed sundials that used the moving shadows cast by the sun through the day to measure times smaller than a day. Today, highly accurate clocks can measure times less than a billionth of a second. The study of time measurement is horology.
The SI (International Systems of Units) unit of time is one second, written as s.
In Einsteinian physics, time and space can be combined into a single concept. See space-time continuum.