  We present and analyze Raman spectra of the Mott insulator Ca$_2$RuO$_4$,
whose quasi-two-dimensional antiferromagnetic order has been described as a
condensate of low-lying spin-orbit excitons with angular momentum $J_{eff}=1$.
In the $A_g$ polarization geometry, the amplitude (Higgs) mode of the
spin-orbit condensate is directly probed in the scalar channel, thus avoiding
infrared-singular magnon contributions. In the $B_{1g}$ geometry, we observe a
single-magnon peak as well as two-magnon and two-Higgs excitations. Model
calculations using exact diagonalization quantitatively agree with the
observations. Together with recent neutron scattering data, our study provides
strong evidence for excitonic magnetism in Ca$_2$RuO$_4$ and points out new
perspectives for research on the Higgs mode in two dimensions.
