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 WordNet 3.0 & Open English WordNet

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time, to accommodate the difference between precise time and imprecise observed solar time, which varies due to irregularities and long-term slowdown in the Earth's rotation. Wikipedia

Extra second inserted or removed to keep civil time in sync with the Earth's rotation Wikidata

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. Wiktionary

Added second. Wiktionary (translation)