leap second

noun

1. 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.

noun

1. a second added to or removed from a scale for reckoning time on one particular occasion, to synchronize it with another scale

1. 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. See more at coordinated universal time.

A second inserted into the year to make up for the fact that the Earth’s rotation is slowing down.