  We propose a non-perturbative and gauge invariant derivation of the static
potential between a heavy-quark ($Q$) and an anti-quark ($\bar{Q}$) at finite
temperature. This proper potential is defined through the spectral function
(SPF) of the thermal Wilson loop and can be shown to satisfy the
Schr\"{o}dinger equation for the heavy $Q\bar{Q}$ pair in the thermal medium.
In general, the proper potential has a real and an imaginary part,corresponding
to the peak position and width of the SPF. The validity of using a
Schr\"{o}dinger equation for heavy $Q\bar{Q}$ can also be checked from the
structure of the SPF. To test this idea, quenched QCD simulations on
anisotropic lattices ($a_\sigma=4a_\tau=0.039\rm fm$, $N^3_\sigma \times
N_{\tau} =20^2 \times (96-32)$) are performed. The real part of the proper
potential below the deconfinement temperature ($T=0.78T_c$) exhibits the well
known Coulombic and confining behavior. At ($T=2.33T_c$) we find that it
coincides with the Debye screened potential obtained from Polyakov-line
correlations in the color-singlet channel under Coulomb gauge fixing. The
physical meaning of the spectral structure of the thermal Wilson loop and the
use of the maximum entropy method (MEM) to extract the real and imaginary part
of the proper potential are also discussed.
