Orthogonal Functions

Two functions $f(x)$ and $g(x)$ are orthogonal on the interval $a \leq x \leq b$ if

$$\left\langle{f(x)\vert g(x)}\right\rangle{} \equiv \int^b_a f(x)g(x)\,dx = 0.$$

See also Orthogonal Polynomials, Orthonormal Functions