essential

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Constituting or being part of the essence of something; inherent.
2. adj. Basic or indispensable; necessary: essential ingredients. See Synonyms at indispensable.
3. adj. Medicine Of, relating to, or being a dysfunctional condition or a disease whose cause is unknown: essential hypertension.
4. adj. 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: essential amino acids.
5. n. Something fundamental.
6. n. Something necessary or indispensable.

Century Dictionary and Cyclopedia

1. Involved in the essence, definition, or nature of a thing or of a word: as, an essential character; an essential quality.
2. Constituting or making that which is characteristic or most important in a thing; fundamental; indispensable: as, an essential feature of Shakspere's style.
3. Specifically, in medicine, idiopathic, not symptomatic merely.
4. Pertaining to or proceeding from an essence; of the nature of an essence or extract.
5. n. Existence; being.
6. n. A fundamental or constituent principle; a distinguishing characteristic.
7. 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.

Wiktionary

1. adj. Necessary.
2. adj. Very important; of high importance.
3. adj. Being in the basic form; showing its essence.
4. adj. 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.
5. adj. medicine Idiopathic.
6. n. A necessary ingredient.
7. n. A fundamental ingredient.

GNU Webster's 1913

1. adj. Belonging to the essence, or that which makes an object, or class of objects, what it is.
2. adj. Hence, really existing; existent.
3. adj. Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.
4. adj. Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed.
5. adj. (Mus.) Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.
6. adj. (Med.) Idiopathic; independent of other diseases.
7. n. obsolete Existence; being.
8. n. That which is essential; first or constituent principle.

WordNet 3.0

1. adj. being or relating to or containing the essence of a plant etc
2. adj. of the greatest importance
3. adj. basic and fundamental
4. adj. absolutely necessary; vitally necessary
5. adj. defining rights and duties as opposed to giving the rules by which rights and duties are established
6. n. anything indispensable