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.
Isosurfaces are normally displayed using computer graphics, and are used as data visualization methods in computational fluid dynamics (CFD), allowing engineers to study features of a fluid flow (gas or liquid) around objects, such as aircraft wings. An isosurface may represent an individual shock wave in supersonic flight, or several isosurfaces may be generated showing a sequence of pressure values in the air flowing around a wing. Isosurfaces tend to be a popular form of visualization for volume datasets since they can be rendered by a simple polygonal model, which can be drawn on the screen very quickly.
A popular method of constructing an isosurface from a data volume is the marching cubes algorithm, and another, very similar method is the marching tetrahedrons algorithm. Yet another is called the asymptotic decider.
See also 
- Charles D. Hansen; Chris R. Johnson (2004). Visualization Handbook. Academic Press. pp. 7–11. ISBN 978-0-12-387582-2.