Adjustment made in time, spring back

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

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.