Let $V$ together with $\langle\cdot,\cdot\rangle$ be a \reftext{def:complex-inner-product-space-2026a}{complex inner product space} and let $\lVert\cdot\rVert$ be the \reftext{def:inner-product-norm-2026a}{norm induced by the inner product}.

A vector $v\in V$ is a \textbf{unit vector} if $\lVert v\rVert=1$.