1. subtonic - Noun
2. subtonic - Adjective
Applied to, or distinguishing, a speech element consisting of tone, or proper vocal sound, not pure as in the vowels, but dimmed and otherwise modified by some kind of obstruction in the oral or the nasal passage, and in some cases with a mixture of breath sound; -- a term introduced by Dr. James Rush in 1833. See Guide to Pronunciation, //155, 199-202.
A subtonic sound or element; a vocal consonant, as b, d, g, n, etc.; a subvocal.
The seventh tone of the scale, or that immediately below the tonic; -- called also subsemitone.
Source: Webster's dictionaryThe degrees of the natural minor scale have the same names as those of the major scale, except the seventh degree, which is known as the subtonic because it is a whole step below the tonic. Source: Internet
Therefore, the seventh scale degree becomes a subtonic to the tonic because it is now a whole tone lower than the tonic, in contrast to the seventh degree in the major scale, which is a semitone tone lower than the tonic ( leading-tone ). Source: Internet
If the subtonic is a semitone away from the tonic, then it is usually called the leading-tone (or leading-note); otherwise the leading-tone refers to the raised subtonic. Source: Internet