A second sometimes inserted at the end of a calendar year to keep atomic clock time in close agreement with the rotation of the Earth, which is gradually slowing.
A second which may be added or subtracted to adjust UTC at either, both, or neither, of two specific opportunities each year. 
[n] - 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
noun 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