A contour line or isoline of a function of two variables is a curve along which the function has a constant value.
It is a cross-section of the three-dimensional graph of the function f(x, y) parallel to the x, y plane.
Contour lines are used e.g. in geography and meteorology.
In cartography, a contour line joins points of equal elevation (height) above a given level, such as mean sea level.