Precalculus (6th Edition) Blitzer

If $\mathbf{v}\cdot \mathbf{w}=0$, then the vectors $\mathbf{v}$ and $\mathbf{w}$ are orthogonal.
Dot product of two vectors is given as $\mathbf{v}\cdot \mathbf{w}=\left| \mathbf{v} \right|\left| \mathbf{w} \right|\cos \theta$ where $\theta$ is the angle between the vectors $\mathbf{v}$ and $\mathbf{w}$. The dot product between $\mathbf{v}$ and $\mathbf{w}$ is zero if the angle between them is $90{}^\circ$, which implies that the vectors are orthogonal to each other.