sub-octave
/ˌsʌbˈɒktɪv/ /ˌsʌbˈɒkteɪv/noun
1The octave below a given note; (also) an organ stop sounding a note one octave below the ordinary pitch, or at intervals of an octave below this (sometimes in first sub-octave, second sub-octave, etc.).
2In full suboctave coupler. A device in an organ or similar instrument which enables a note one octave lower than that being played to sound at the same time.
Origin
Mid 17th century; earliest use found in Christopher Simpson (c1602–1669), composer and musician. From sub- + octave.