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Related to leap second: leap year
A second of time, as measured by an atomic clock, added to or omitted from official timekeeping systems periodically to compensate for small changes in the rotation of the earth and therefore the length of a solar day.
(Units) a second added to or removed from a scale for reckoning time on one particular occasion, to synchronize it with another scale
an extra second intercalated into the world's timekeeping system about once a year, made necessary by the gradual slowing down of the earth's rotation.