Texture. The combination of instruments and / or voices.
Texture Solo – A single voice or instrument.
Unison – More than one instrument playing the same music at the same time.
Monophonic - One melody line playing without harmony. (Can be in octaves).
Homophonic/chordal – More than one part moving in chords / harmony.
Polyphonic - Two or more different melodies playing at the same time. (Eg contrapuntal / counterpoint)
Parallel motion – Two (or more) parts moving in the same direction.
Contrary motion – Two (or more) parts moving in the opposite direction.
Melody and Accompaniment – A clear tune being supported by other instruments.
Heterophonic (Gamelan / Irish Folk) – Different versions of the same melody played at the same time.
Thick / Thin – Different ways to describe dense or sparse textures.