length of day (LOD)

common term for the difference in the duration of a day as measured by UT1 and 86,400 SI seconds. In practice this quantity is determined by differencing daily values of UT1-UTC. Units are generally given as ms day^-1.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)