A necessary ingredient.
A fundamental ingredient.
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.
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.