For any function f:
f is an even function is f(−x) = f(x) for all x (symmetrical about the vertical y axis).
f is an odd function if f(−x) = −f(x) for all x (symmetrical about the line formed by the function y = −x).
For example, y = x2 is an even function ( f(−2) = 4 = f(2)) and y = x3 is an odd function ( f(−2) = −8 = −f(2)).