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.
Origin
1970-75
British Dictionary definitions for leap seconds

leap second

noun
1.
a second added to or removed from a scale for reckoning time on one particular occasion, to synchronize it with another scale
leap seconds in Science
leap second
  (lēp)   
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.
leap seconds in Culture

leap second definition


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.