An Isosurface is an N-dimensional surface extracted from an N-dimensional volume where a specified field is of a specified value, often called the isovalue. In simpler terms, an isosurface is a three-dimensional (3-D) polygonal surface extracted from a 3-D volume where the data is a single value.
Often times the polygonal data is easier to understand and manipulate than the source volumetric data. Isosurfaces fall into two main categories.
In three dimensions, a polygonal isosurface is extracted from a 3-D volume. One commonly seen case of this is in medical imagery where a single density value (representing bone) is extracted from a CT or MRI scan. This will reconstruct a 3-D model of the bone that can be analyzed to find damage or deformities.
In two dimensions, a line is extracted from a 2-D volume. The line is commonly referred to as an isoline or contour line. One commonly known example of this is in cartography. Topographic maps show specific elevations as lines across the 2-D surface. Each Contour Line indicates a specific elevation value, with multiple Contour Lines shown simultaneously.