"It was Riemann who coined the term 'function' in Vereinfachte Harmonielehre (1893) to describe relations between the dominant and subdominant harmonies and the referential tonic: he borrowed the word from mathematics, where it was used to designate the correlation of two variables, an 'argument' and a 'value'". Brian Hyer, "Tonality", Grove Music Online, doi:10.1093/gmo/9781561592630.article.28102.
