Leading Tone Scale Degree
From bib. source
So, for example, for the C Major scale, the seventh scale degree would be B:

Note
By “a scale degree under the tonic,¨ the quote means counting the degrees in the reverse direction.
music music_theory leading_tone scale_degree
bibliography
- “The Leading Tone: The 7th 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., 65. Minneapolis, MN: Slam Academy, 2018.