Essential

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