  We report the first measurement of coherent elastic neutrino-nucleus
scattering (\cevns) on argon using a liquid argon detector at the Oak Ridge
National Laboratory Spallation Neutron Source. Two independent analyses prefer
\cevns over the background-only null hypothesis with greater than $3\sigma$
significance. The measured cross section, averaged over the incident neutrino
flux, is (2.2 $\pm$ 0.7) $\times$10$^{-39}$ cm$^2$ -- consistent with the
standard model prediction. The neutron-number dependence of this result,
together with that from our previous measurement on CsI, confirms the existence
of the \cevns process and provides improved constraints on non-standard
neutrino interactions.
