1. (a.) Belonging to the essence, or that which makes an object, or class of objects, what it is.

2. (a.) Hence, really existing; existent.

3. (a.) Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.

4. (a.) Containing the essence or characteristic portion of a substance, as of a plant; highly rectified; pure; hence, unmixed; as, an essential oil.

5. (a.) Necessary; indispensable; -- said of those tones which constitute a chord, in distinction from ornamental or passing tones.

6. (a.) Idiopathic; independent of other diseases.