I . a.
1.
Vital, necessary, requisite, indispensable, highly important.
2.
Volatile, diffusible, pure, highly rectified.
3.
( Med. ) Idiopathic.
II . n.
[ Usually in pl. ]
1.
Indispensable element, vital part, absolute requisite.
2.
Rudiment, first principle, first step, outline.