Meaning

• A necessary ingredient.

• A fundamental ingredient.

Meaning

• Necessary.

• Very important; of high importance.

• Necessary for survival but not synthesized by the organism, thus needing to be ingested

• Being in the basic form; showing its essence.

  "Don’t mind him being grumpy. That’s the essential Fred."

• Really existing; existent.

• Such that each complementary region is irreducible, the boundary of each complementary region is incompressible by disks and monogons in the complementary region, and no leaf is a sphere or a torus bounding a solid torus in the manifold.

• Idiopathic.