Level curve
From Conservapedia
In mathematics, a level curve is the curve formed by taking all the values of some functions 
such that the function is some constant value c. A set of level curves for different constants c is referred to as a contour plot.
These are also referred to as isocurves of the function. For functions of three variables, the terms level surface or isosurface are used.
The value of a function is identical at every point along any of its level curves. For example, the level curve of the paraboloid Z(x,y) = x2 + y2 at Z=4 is the circle x2 + y2 = 4. Therefore, the gradient of a function (which represents the rate of fastest change) is always perpendicular to its level curves because it is a vector that takes the direction of maximum increase in f.
