opposite

Given a right triangle with an acute angle $\theta$, the side of the triangle that is opposite of $\theta$ is the side of the triangle that is not a side (http://planetmath.org/Angle) of $\theta$.

This definition can be generalized to any triangle and any angle $\theta$: The side of the triangle that is opposite of $\theta$ is the side of the triangle that is not a side of $\theta$. This more general definition is not used as commonly as the definition for right triangles.