  We report measurements of optical absorption in the zig-zag antiferromagnet
$\alpha$-RuCl$_3$ as a function of temperature, $T$, magnetic field, $B$, and
photon energy, $\hbar\omega$ in the range $\sim$ 0.3 to 8.3 meV, using
time-domain terahertz spectroscopy. Polarized measurements show that 3-fold
rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. We find
a sharp absorption peak at 2.56 meV upon cooling below the N\'eel temperature
of 7 K at $B=0$ that we identify as magnetic-dipole excitation of a
zero-wavevector magnon, or antiferromagnetic resonance (AFMR). With application
of $B$, the AFMR broadens and shifts to lower frequency as long-range magnetic
order is lost in a manner consistent with transitioning to a spin-disordered
phase. From direct, internally calibrated measurement of the AFMR spectral
weight, we place an upper bound on the contribution to the $dc$ susceptibility
from a magnetic excitation continuum.
