'A440' originates from combining the note name 'A' with the frequency '440 Hz', where 'Hz' refers to 'Hertz' (named after the German physicist Heinrich Hertz).

'A' as a musical note name stems from medieval pitch naming, while numeric frequency notation became standard after the adoption of 'Hertz' as the unit of frequency in the 19th century; the label 'A440' emerged in the 20th century and was adopted as a widely used concert-pitch standard by national and international organizations.

Initially, such notations simply indicated the measured frequency of a given A; over time 'A440' evolved to denote the accepted international concert-pitch standard and the reference tone used for tuning.