essential vs internal

essential

adj
  • Having the nature of essence; not physical. 

  • Necessary. 

  • 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. 

  • Very important; of high importance. 

  • 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. 

noun
  • A necessary ingredient. 

  • A fundamental ingredient. 

internal

adj
  • Of the inner nature of a thing. 

  • Within the body. 

  • Concerned with the domestic affairs of a nation, state or other political community. 

  • Applied or intended for application through the stomach by being swallowed. 

  • Concerned with the non-public affairs of a company or other organisation. 

  • Attending a university as well as taking its examinations. 

  • Of or situated on the inside. 

  • Experienced in one's mind; inner rather than expressed. 

  • Present or arising within an organism or one of its parts. 

How often have the words essential and internal occurred in a corpus of books? (source: Google Ngram Viewer )