Indispensable (adjective)

1. Absolutely necessary; essential.
2. Absolutely required in order for something to happen or be done.
3. Absolutely required for a particular purpose.

Origin:

From latin indispensabilis, from in- ("not") + dispensabilis ("dispensabl.e")

Examples:

1. He was an indispensable member of the team.
2. The tool was indispensable for the job.
3. The book was an indispensable reference for the research.
4. She was an indispensable source of information.
5. The software was indispensable for the organization.