T eph (T_eph)

independent time argument of JPL ephemerides (Standish, A&A, 336, 381, 1998) that is, for practical purposes, the same as Barycentric Dynamical Time (TDB). TDB is related to Barycentric Coordinate Time (TCB) by
TDB = TCB - L_B × (JD_TCB - T₀) × 86400 + TDB₀, where T₀ = 2443144.5003725, and
L_B = 1.550519768 × 10^-8 and TDB₀ = -6.55 × 10^-5 s
are defining constants. [IAU 2006 Resolution B3]

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