From AMS Glossary
A constant-entropy chart; a synoptic chart presenting the distribution of meteorological elements in the atmosphere on a surface of constant potential temperature (equivalent to an isentropic surface).
It usually contains the plotted data and analysis of such elements as pressure (or height), wind, temperature, and moisture at that surface.
Saucier, W. J. 1955. Principles of Meteorological Analysis. chap. 8.