| 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. |
| leap second n. 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. |
A second inserted into the year to make up for the fact that the Earth's rotation is slowing down.
Note: Scientists know when to insert a leap second by comparing the Earth's rotation to an atomic clock.
leap second
Coordinated Universal Time