From AMS Glossary
(Also called constant-level chart, fixed-level chart, isohypsic chart.) A synoptic chart for any surface of constant geometric altitude above mean sea level (a constant- height surface), usually containing plotted data and analyses of the distribution of such variables as pressure, wind, temperature, and humidity at that altitude.
A commonly analyzed constant-height chart is the surface chart (or "sea level chart"). All of the upper-air charts are considered constant-pressure charts.