What does essential mean?

According to American Heritage Stedman's Medical Dictionary

the definition of essential is:

1. Constituting or being part of the essence of something; inherent.
2. Basic or indispensable; necessary.
3. Of, relating to, or being a dysfunctional condition or a disease whose cause is unknown.
4. Of, relating to, or being a substance that is required for normal functioning but cannot be synthesized by the body and therefore must be included in the diet.

n.

1. Something fundamental.
2. Something necessary or indispensable. to much infow