Hilbert space

s were a mathematical concept due to Hilbert. (PROSE: Christmas on a Rational Planet)

Behind the scenes
A Hilbert space is an infinite dimensional, where given two elements of the space you can uniquely map them to a number up to (ie, the "projection" of one element onto another) and there are no "holes" in the space. (For example, the rational numbers would be a space with "holes" while the reals would not.)