organum

noun, plural or·ga·na [awr-guh-nuh] /ˈɔr gə nə/, or·ga·nums.

1. an organon.
2. Music.
   1. the doubling, or simultaneous singing, of a melody at an interval of either a fourth, a fifth, or an octave.
   2. the second part in such singing.

noun plural -na (-nə) or -nums

1. a form of polyphonic music originating in the ninth century, consisting of a plainsong melody with parts added at the fourth and fifth
2. a variant of organon