Necessary (adjective)

1. Absolutely needed or required.
2. Of the nature of a necessity.
3. Inevitable or likely to happen.

Origin:

Late middle english: from old french necessaire, from latin necessarius, from necesse "needful".

Examples:

1. It is necessary to complete the task before the deadline.
2. It is necessary to bring your ID to enter the building.
3. Additional funding is necessary to continue the research.
4. It is necessary to maintain the equipment.
5. The surgery was necessary to save his life.