Coming at this from a mathematical viewpoint, a Hilbert Space is a complete inner product space. See, for example:
https://en.wikipedia.org/wiki/Hilbert_space
In QM the Hilbert Space is often a space of square-integrable complex-valued functions. But, for Spin 1/2 particles, for example. the Hilbert space is good old two-dimensional complex vectors and operators are 2x2 matrices.
I don't really understand why some physics authors talk about "Hilbert Space", as though it were a single entity, given that different QM scenarios have different Hilbert Spaces associated with them.