Orthogonality

State ∣u⟩,∣v⟩\left\lvert u \right\rangle,\left\lvert v \right\rangle are orthogonal to each other if the Inner product of state ∣u⟩\left\lvert u \right\rangle and ∣v⟩\left\lvert v \right\rangle are zero.

∣u⟩⋅∣v⟩=0\begin{gather*} \left\lvert u \right\rangle\cdot \left\lvert v \right\rangle=0 \end{gather*}

They’re also known as being perpendicular!