Essential

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

Hence, really existing; existent.

Important in the highest degree; indispensable to the attainment of an object; indispensably necessary.

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

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

Idiopathic; independent of other diseases.