From AMS Glossary
(Also called contour, isohypse, isoheight.) A line of constant elevation above a certain reference level (usually mean sea level) on a previously defined surface, which may be the earth's surface, a constant-pressure surface, an isentropic surface, etc.
A contour line of a given value is the intersection of the surface in question with the constant- height surface of the same elevation as the value of the contour line. In meteorology, a contour line frequently refers to a line of constant height on a constant-pressure chart.