Definition of medius in English:

medius

Pronunciation /ˈmiːdɪəs/

noun

historical Music
  • An intermediate vocal part in polyphonic music, often an alto or second (lower) treble part; = "mean".

Origin

Mid 16th century; earliest use found in Thomas Sternhold (d. 1549), author of metrical psalm paraphrases. From classical Latin medius middle, after mean.