The second is the SI base unit for time and has the symbol s.
The second is commonly understood to be 1/86400 of a day; there are 60 seconds in a minute, 60 minutes in an hour and 24 hours in a day.
Analog and digital watches and clocks almost all measure and display the progression of time via the use of a second counter / hand - and is generally considered the lowest denomination of time.
The earliest display and use of seconds was in the last half of the 16th century. Prior to this, it was not considered accurate enough to measure in seconds as a mechanical device was needed to ensure consistency.
In 1656, a Dutch scientist invented the first pendulum clock that measured seconds. His name was Christiaan Huygens.