1. A necessary ingredient.
  2. A fundamental ingredient.


  1. Necessary.
  2. Very important; of high importance.
  3. Being in the basic form; showing its essence.
    Don’t mind him being grumpy. That’s the essential Fred.
  4. 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.
  1. Idiopathic.

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