Semi-Hilbert space

A semi-Hilbert space is a generalization of the concept of a Hilbert space, which is a complete inner product space. While a Hilbert space has a complete inner product structure, a semi-Hilbert space maintains some of the properties of a Hilbert space but may not be complete. In a semi-Hilbert space, one can still define an inner product, which allows for the measurement of angles and distances.