What is the definition of a second?

Originally based on the Earth's rotation this has been found to be irregular and has in more recent times been replaced by atomic time.

The overhead sun at noon can arrive 16 minutes 18 seconds early or up to 14 minutes 28 seconds late. In fact only 4 days per year are precisely 24 hours long as measured by the Sun. These days occur on, or about 25 December, 15 April, 14 June and 31 August. The remaining days are longer or shorter as measured from the overhead midday sun.

A more precise measure of time was required. So there have been many definitions of the second throughout time:

History

1884 - International Meridian Conference (Washington DC): based on a mean solar day at Greenwich, England.
1956 - Ephemeris Time: Based on lunar observations
1958 - Atomic Time: Based on clock data from numerous countries
1967 - Atomic Time: based on radiation patterns of the caesium element.
1972 - Co-ordinated Universal Time (UTC) introduced.
1984 - Dynamical time - based on motion of celestial bodies
1986 - UTC replaced GMT.

Despite time being measured highly accurately by atomic means the earth time stills rules. Should earth time and atomic time get out of step scientists adjust time by subtracting or adding "leap seconds" on the last day of June or December.

Confused? - 300 years of history still means that most people regard Greenwich Mean Time (GMT) as the standard for their everyday lives.