Wiktionary
isosurface
n. (context computer graphics English) A surface that represents points of a constant value within a volume of space
Wikipedia
Isosurface
An isosurface is a three-dimensional analog of an isoline. It is a surface that represents points of a constant value (e.g. pressure, temperature, velocity, density) within a volume of space; in other words, it is a level set of a continuous function whose domain is 3D-space.