Isohypse is correct. It means equal or uniform (iso-, from Greek iso, isos: equal) height (hyps, from Greek hupsos: height or top). The study of the topography of the earth's surface, particularly its varying elevation, is hypsography. The practice of determining elevation points is hypsometry. The colors of an elevation map are called hysometric tints (may I say what a joy it has been to work for more than 20 years with cartographers who use that expression routinely?), and such a map itself is formally termed a hypsometric map. Because that term is obscure, most such maps are called topographic, elevation, or simply physical maps. Topographic maps, by the way, are any that convey the surface features of the map area, whether with contour lines, hypsometric tints, relief shading, pictographic symbols, or any other means.
Found the citations for isohypse itself; also recasting references for the related terms since the original format made them easily misunderstood.
isohypse Longman Dictionary of Geography, Audrey N. Clark, 1985 (London: Longman) and Glossary of Geology, American Geological Institute, 1972 (Washington, D.C.: American Geological Institute).
hypsometry and hypsography American Heritage Dictionary of the English Language, 4e, 2000 (Boston: Houghton Mifflin).
hypsometric map GIS Dictionary, Esri
hypsometric tints "The Development and Rationale of Cross-blended Hypsometric Tints," T. Patterson and B. Jenny, in Cartographic Perspectives, Number 69, 2011.