Barycentric Dynamical Time (TDB)
a time scale originally intended to serve as an independent time argument of barycentric ephemerides and equations of motion. In the IAU 1976 resolutions, the difference between TDB and TDT was stipulated to consist of only periodic terms, a condition that cannot be satisfied rigorously. The IAU 1991 resolutions introducing barycentric coordinate time (TCB) noted that TDB is a linear function of TCB, but without explicitly fixing the rate ratio and zero point, leading to multiple realizations of TDB. In 2006 TDB was re-defined through the following linear transformation of TCB (IAU 2006 Resolution B3):
TDB = TCB − LB x ( JDTCB − T0 ) x 86400 + TDB0'
where T0 = 2443144.5003725, and LB = 1.550519768x10-8 and
TDB0 = − 6.55x10-5 s are defining constants.
Source: 
Glossary (PDF, 237KB) of the IERS Conventions (2010)
Original Source: IAU 2006 NFA Glossary
