instrumental music
Refers to music performed solely on instruments, without vocal parts. Often contrasted with 声楽 (vocal music).
He specializes in composing instrumental music.
The orchestra performed instrumental music.
I'm taking an instrumental music class at junior high school.
楽器 means 'musical instrument' (a physical object), whereas 器楽 means 'instrumental music' (a genre of performance); the kanji are simply reversed, so learners often swap them.
instrumental piece; piece of instrumental music
Compound of 器 (instrument) and 楽 (music). The exact historical derivation is uncertain, but the term is a standard modern word for instrumental music.