Sentence:  Functional analysis is best approached through a sound knowledge of Hilbert <WSD>space</WSD> theory. 
Meaning of the word marked with <WSD> tags:  A generalized construct or set whose members have some property in common; typically there will be a geometric metaphor allowing these members to be viewed as "points". Often used with a restricting modifier describing the members (e.g. "vector space"), or indicating the inventor of the construct (e.g. "Hilbert space"). (from 20thc.)