essential

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Constituting or being part of the nature or essence of something; inherent.
• adjective Fundamentally important or necessary: synonym: indispensable.
• adjective Medicine Of, relating to, or being a condition or a disease whose cause is unknown.
• adjective Biochemistry Being a substance that is required for normal functioning but cannot be synthesized by the body and therefore must be included in the diet.
• noun Something fundamental.
• noun Something necessary or indispensable.

from The Century Dictionary.

• Involved in the essence, definition, or nature of a thing or of a word: as, an essential character; an essential quality.
• Constituting or making that which is characteristic or most important in a thing; fundamental; indispensable: as, an essential feature of Shakspere's style.
• Specifically, in medicine, idiopathic, not symptomatic merely.
• Pertaining to or proceeding from an essence; of the nature of an essence or extract.
• noun Existence; being.
• noun A fundamental or constituent principle; a distinguishing characteristic.
• In geology, noting those minerals of any species of rock which establish its character and which must be mentioned in its definition: contrasted with accessory.

from the GNU version of the Collaborative International Dictionary of English.

• noun obsolete Existence; being.
• noun That which is essential; first or constituent principle.
• adjective Belonging to the essence, or that which makes an object, or class of objects, what it is.
• adjective Hence, really existing; existent.
• adjective Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.
• adjective Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed.
• adjective (Mus.) Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.
• adjective (Med.) Idiopathic; independent of other diseases.
• adjective (Biol.) the prominent characteristics which serve to distinguish one genus, species, etc., from another.
• adjective (Med.) one that is not dependent on another.
• adjective (Chem.) a class of volatile oils, extracted from plants, fruits, or flowers, having each its characteristic odor, and hot burning taste. They are used in essences, perfumery, etc., and include many varieties of compounds; as lemon oil is a terpene, oil of bitter almonds an aldehyde, oil of wintergreen an ethereal salt, etc.; -- called also volatile oils in distinction from the fixed or nonvolatile.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective Necessary.
• adjective Very important; of high importance.
• adjective Being in the basic form; showing its essence.
• 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 medicine Idiopathic.
• noun A necessary ingredient.
• noun A fundamental ingredient.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective being or relating to or containing the essence of a plant etc
• adjective of the greatest importance
• adjective basic and fundamental
• adjective absolutely necessary; vitally necessary
• adjective defining rights and duties as opposed to giving the rules by which rights and duties are established
• noun anything indispensable