essential

basic, essential, fundamental, radical, vital

indispensable, essential, necessary, requisite

inherent, essential

名词 essential:

1. anything indispensable
   同义词：necessity, requirement, requisite, necessary

形容词 essential:

1. absolutely necessary; vitally necessary
   同义词：indispensable
2. basic and fundamental
3. of the greatest importance
   同义词：all-important, all important, crucial, of the essence
4. being or relating to or containing the essence of a plant etc
5. defining rights and duties as opposed to giving the rules by which rights and duties are established
   同义词：substantive
6. absolutely required and not to be used up or sacrificed

1. In considering this problem, you should grasp its essentials.
   在考虑这一问题时，你应当抓住实质。
2. Her most essential quality is kindness.
   她最主要的品质是善良。
3. Food is essential to life.
   食物对生命是必不可少的。

es.sen.tial

adj.（形容词）
Constituting or being part of the essence of something; inherent.
本质的：是物体必不可少的组成部分；固有的
Basic or indispensable; necessary:
基本的，必须的：基本的或不可缺少的；绝对必要的：
例句：
essential ingredients.See Synonyms at indispensable
必要的构成要素参见 indispensable

Medicine Of, relating to, or being a dysfunctional condition or a disease whose cause is unknown:
【医学】 原发的：疾病或状态自发的，原发的或特发的：
例句：
essential hypertension.
本态性高血压

Biochemistry Being a substance that is required for normal functioning but cannot be synthesized by the body and therefore must be included in the diet.
【生物化学】 元素：人体某一功能所必需，又不能由身体合成的物质，必须由食物含这种物质
n.（名词）
Something fundamental.
基本要素：基本必要的东西
Something necessary or indispensable.
必需品：某物要素或实质

【引伸】
essen.tial.ity
essen.tially
adv.（副词）