A leap second is an intercalary, one-second adjustment that keeps broadcast standards for time of day close to mean solar time. Broadcast standards for civil time are based on Coordinated Universal Time (UTC), a time standard which is maintained using extremely precise atomic clocks. To keep the UTC broadcast standard close to mean solar time, UTC is occasionally corrected by an intercalary adjustment, or "leap", of one (1) second. Over long time periods, leap seconds must be added at an ever increasing rate (see ΔT). The name is based on the term "leap years", though in doing so it is a little inconsistent: leap seconds result in an extra second, while leap years result in an extra day, not an extra year
A second of time, as measured by an atomic clock, added to or omitted from official timekeeping systems annually to compensate for changes in the rotation of the earth