a.

1. 必要的，不可缺的[（+to/for）]
   Hard work is essential to success.

   成功必須努力工作。

2. 本質的，實質的；基本的[B]
   There is no essential difference between the two drafts.

   兩份草稿沒有本質上的不同。

3. 提煉的，精華的

n.[P]

1. 要素，要點；必需品
   Our course deals with the essentials of management.

   我們的課程講述管理的基本要點。

2. 本質，實質

adj.

1. absolutely necessary; extremely important.
2. central to the nature of something; fundamental:
   the essential weakness of the plaintiff's case.
3. (of an amino acid or fatty acid) required for normal growth but not synthesized in the body and therefore necessary in the diet.
4. Medicine  (of a disease) with no known external stimulus or cause; idiopathic.

n.

1. things that are absolutely necessary.
2. the fundamental elements of something.