  The rare-earth triangular-lattice chalcogenide is a great platform for
exploring both spin liquids and novel magnetic orders with anisotropic spin
interactions and magnetic frustrations. Here, we report the thermodynamic and
neutron scattering measurements of rare-earth triangular-lattice chalcogenide
KErSe$_{2}$, using single-crystal samples. Our experiments revealed a
long-range stripe order below 0.2 K. Although the magnetic order was
three-dimensional, magnetic excitations exhibited negligible modulation along
the z direction, indicating very weak interlayer coupling. Furthermore,
magnetic excitation developed a well-defined spin-wave dispersion with a gap of
$\sim$0.03 meV at M points. Both the stripe order and spin-wave excitations
could be quantitatively understood from the anisotropic spin interactions of
the Er$^{3+}$ Kramers doublets.
