  The spin waves in single crystals of the layered van der Waals
antiferromagnet CoPS$_3$ have been measured using inelastic neutron scattering.
The data show four distinct spin wave branches with large ($\gtrsim 14$ meV)
energy gaps at the Brillouin zone center indicating significant anisotropy. The
data were modelled using linear spin wave theory derived from a Heisenberg
Hamiltonian. Exchange interactions up to the third nearest-neighbour in the
layered planes were required to fit the data with ferromagnetic $J_1 = -1.37$
meV between first neighbours, antiferromagnetic $J_3 = 3.0$ meV between third
neighbours, and a very small $J_2 = 0.09$ meV between second neighbours. A
biaxial single-ion anisotropy was required, with a collinear term $D^x = -0.77$
meV for the axis parallel to the aligned moment direction and a coplanar term
$D^z=6.07$ meV for an axis approximately normal to the layered crystal planes.
