  The chain-like $ZrTe_3$ compound undergoes a charge-density-wave (CDW)
transition at $T_{CDW}=63$ $K$, most strongly affecting the conductivity
perpendicular to the chains. We measure the temperature ($T$) dependence of the
optical reflectivity from the far infrared up to the ultraviolet with polarized
light. The CDW gap $\Delta(T)$ along the direction perpendicular to the chains
is compatible for $T<T_{CDW}$ with the behavior of an order parameter within
the mean-field Bardeen-Cooper-Schrieffer (BCS) theory. $\Delta(T)$ also
persists well above $T_{CDW}$, which emphasizes the role played by fluctuation
effects.
