# Unitary Operator A [Unitary Operator](https://en.wikipedia.org/wiki/Unitary_transformation) is a transform that preserves the inner product. In other words: > The inner product of two vectors before the transformation is equal to their inner product after the transformation. More precisely we can say that a Unitary Operator is an [Isomorphism](Isomorphism.md) between two [Hilbert-Space](Hilbert-Space.md).