Terrestrial Intermediate Reference System (TIRS)
a geocentric reference system defined by the intermediate equator of the CIP and the TIO (IAU 2006 Resolution B2). It is related to the ITRS by polar motion and s′ (see TIO locator). It is related to the Celestial Intermediate Reference System by a rotation of ERA around the CIP, which defines the common z-axis of the two systems. Since the acronym for this system is close to another acronym (namely ITRS), it is suggested that wherever possible the complete name be used.
Source: 
Glossary (PDF, 237KB) of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
