  We present a far-infrared magneto-optical study of the gapped nodal-line
semimetal ZrSiS in magnetic fields $B$ up to 7 T. The observed field-dependent
features, which represent intra- (cyclotron resonance) and interband
transitions, develop as $\sqrt{B}$ in increasing field and can be consistently
explained within a simple 2D Dirac band model with a gap of 26 meV and an
averaged Fermi velocity of $3\times10^{5}$ m/s. This indicates a rather narrow
distribution of these parameters along the in-plane portions of the nodal line
in the Brillouin zone. A field-induced feature with an energy position that
does not depend on $B$ is also detected in the spectra. Possible origins of
this feature are discussed.
