noun
subdominant
In music theory, the fourth scale degree; the note a perfect fifth below the tonic.
ハ長調の下属音はファです。
The subdominant of C major is F.
属音 is the dominant (fifth scale degree), while 下属音 is the subdominant (fourth scale degree).
Compound of 下 (sub-) + 属 (belonging) + 音 (sound/note), a calque from Western music terminology.