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 = c }, {\displaystyle L_{c}=\left\{\mid f=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. Wikipedia