A type of wave propagated in the superfluid phase of liquid helium (helium II) and in certain other substances under special conditions. Predicted independently by L. Tisza in 1938 and L. Landau in 1941, such waves were observed first by V. Peshkov in 1944 and have subsequently provided a rich source of information for the study and understanding of the superfluid state. The name is misleading since second sound is not in any sense a sound wave, but a temperature or entropy wave. In ordinary or first sound, pressure and density variations propagate with very small accompanying variations in temperature; in second sound, temperature variations propagate with no appreciable variation in density or pressure.