Supertonic Scale Degree
From “Supertonic: The 2nd Scale Degree¨ in Music Theory for Electronic Music Producers
The tonic
You may be curious what the tonic would then be in this case. Since this is referring to what sounds like the home key, often the root key, the tonic likely refers to that key which, in the given musical scale, is often the first degree. Thus, the tonic is the first degree, while the supertonic is the second degree.
In the C Major scale (for more on scales, see 20240904103821-Musical_Scale_and_Key_Patterns), the tonic is the C pitch while the D is the supertonic as the former is the first scale degree of C Major and the latter is the second scale degree of C Major:

scale_degree scale musical_scale music music_theory supertonic home_key pitch pitches keys degree
bibliography
- “Supertonic: The 2nd Scale Degree.” In Music Theory for Electronic Music Producers: The Producer’s Guide to Harmony, Chord Progressions, and Song Structure in the MIDI Grid., 1st ed., 60. Minneapolis, MN: Slam Academy, 2018.