Adjustment made in time, spring back

Wiktionary Word definitions in Wiktionary
n. A second of time added to the year occasionally to compensate for variation in the rate of Earth's rotation relative to the absolute standards of time.

Wikipedia Word definitions in Wikipedia
A leap second is a one- second adjustment that is occasionally applied to Coordinated Universal Time (UTC) in order to keep its time of day close to the mean solar time , or UT1 . Without such a correction, time reckoned by Earth's rotation drifts away ...

WordNet Word definitions in WordNet
n. a second (as measured by an atomic clock) added to or subtracted from Greenwich Mean Time in order to compensate for slowing in the Earth's rotation