  We present a high energy-resolution inelastic x-ray scattering investigation
of the soft phonon mode in the charge-density-wave system TbTe$_3$. We analyze
our data based on lattice dynamical calculations using
density-functional-perturbation-theory and find clear evidence that strongly
momentum dependent electron-phonon-coupling defines the periodicity of the CDW
superstructure: Our experiment reveals strong phonon softening and increased
phonon line widths over a large part in reciprocal space adjacent to the CDW
ordering vector $q_{CDW} = (0, 0, 0.3)$. Further, $q_{CDW}$ is clearly offset
from the wave vector of (weak) Fermi surface nesting $q_{FS} = (0, 0, 0.25)$
and our detailed analysis indicates that electron-phonon-coupling is
responsible for this shift. Hence, we can add TbTe$_3$, which was previously
considered as a canonical CDW compound following the Peierls scenario, to the
list of distinct charge-density-wave materials characterized by momentum
dependent electron-phonon coupling.
