essential

(noun): A necessary ingredient.
(noun): A fundamental ingredient.
(adjective): Necessary.
(adjective): Very important; of high importance.
(adjective): Necessary for survival but not synthesized by the organism, thus needing to be ingested
(adjective): Being in the basic form; showing its essence.
(adjective): Really existing; existent.
(adjective): 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.
(adjective): Idiopathic.