Roton
A roton is an excitation in superfluid Helium-4. A graph of the energy vs. the momentum of such an excitation increases linearly from the origin, but exhibits first a maximum and then a minimum in energy as the momentum increases. Excitations with momenta in the linear region are called phonons; those with momenta close to the minimum are called rotons. Excitations with momenta near the maximum are sometimes called maxons.
Roton also refers to the Roton SSTO spacecraft design.