Extremely important or necessary.
1) Fully aware that he was an indispensable assistant, he decided that it was high time that he was paid for what he was worth for.
2) She is an indispensable part of the committee.
From Medieval Latin indispensabilis, from in- meaning 'not' and dispensabilis.
all-important, critical, imperitive, essential.
dispensable, needless, unnecessary.