Indispensable, essential, imperative, vital.
essential or central; of great importance or relevance.
Pertaining to, made by, or prepared for an instrument, especially a musical instrument (rather than the human voice).
Applied to a case expressing means or agency, generally indicated in English by by or with with the objective.
Serving as an instrument, medium, means, or agency.
The instrumental case.
A composition written or performed without lyrics, sometimes using a lead instrument to replace vocals.