  We report a precise measurement of the parity-violating asymmetry $A_{\rm
PV}$ in the elastic scattering of longitudinally polarized electrons from
$^{48}{\rm Ca}$. We measure $A_{\rm PV} =2668\pm 106\ {\rm (stat)}\pm 40\ {\rm
(syst)}$ parts per billion, leading to an extraction of the neutral weak form
factor $F_{\rm W} (q=0.8733$ fm$^{-1}) = 0.1304 \pm 0.0052 \ {\rm (stat)}\pm
0.0020\ {\rm (syst)}$ and the charge minus the weak form factor $F_{\rm ch} -
F_{\rm W} = 0.0277\pm 0.0055$. The resulting neutron skin thickness
$R_n-R_p=0.121 \pm 0.026\ {\rm (exp)} \pm 0.024\ {\rm (model)}$~fm is
relatively thin yet consistent with many model calculations. The combined CREX
and PREX results will have implications for future energy density functional
calculations and on the density dependence of the symmetry energy of nuclear
matter.
