Essential (adjective)

1. Absolutely necessary; extremely important.
2. Absolutely required or necessary; extremely important.
3. Indispensable; necessary for existence or survival.
4. Fundamental; basic; inherent.
5. Crucial; vital; critical.

Origin:

Late middle english: from old french essentiel or late latin essentialis, from essentia "being, essence" (see essence).

Examples:

1. He is essential to the project.
2. She is essential to the team.
3. They are essential to the success of the company.
4. Air and water are essential for life.
5. This is an essential part of the job.