Level set
In mathematics, a level set of a real-valued function f of n real variables is a set where the function takes on a given constant value c, that is: L c ( f ) = { ( x 1 , … , x n ) ∣ f ( x 1 , … , x n ) = c } . {\displaystyle L_{c}(f)=\left\{(x_{1},\ldots ,x_{n})\mid f(x_{1},\ldots ,x_{n})=c\right\}~.} When the number of independent variables is two, a level set is called a level curve, also known as contour line or isoline; so a level curve is the set of all real-valued solutions of an equation in two variables x1 and x2.