Even function

A function that does not change sign when the sign of the independent variable is changed, i.e. satisfying the condition $f(-x)=f(x)$. The graph of an even function is symmetric about the $y$-axis.

Comments

A function that does change sign when the sign of the independent variable is changed, i.e. satisfying the condition $f(x)=-f(-x)$, is called an odd function.