subtonic

[suhb-ton-ik] /sʌbˈtɒn ɪk/
noun
1.
the seventh tone of a scale, being the next below the upper tonic.
Origin
1825-35; sub- + tonic
British Dictionary definitions for subtonic

subtonic

/sʌbˈtɒnɪk/
noun
1.
(music) the seventh degree of a major or minor scale Also called leading note