1. 绝对必要的;必不可少的;基本的
    Something that is essential is extremely important or absolutely necessary to a particular subject, situation, or activity.

essential在线翻译

    e.g. It was absolutely essential to separate crops from the areas that animals used as pasture...
           将庄稼和放牧区分开绝对必要。
    e.g. As they must also sprint over short distances, speed is essential...
           由于他们也必须进行短跑，因此速度至关重要。

2. 必需品;不可或缺之物
    The essentials are the things that are absolutely necessary for the situation you are in or for the task you are doing.

essential的翻译

    e.g. The flat contained the basic essentials for bachelor life.
           那套公寓配有单身生活的基本必需品。

3. 基本的;本质的；根本的
    The essential aspects of something are its most basic or important aspects.

    e.g. Most authorities agree that play is an essential part of a child's development...
           大多数权威人士都同意，玩耍在孩子成长过程中起着非常重要的作用。
    e.g. In this trial two essential elements must be proven: motive and opportunity.
           在这次审讯中，必须证明两个基本因素：动机和时机。

4. （某学科的）基础，基本知识
    The essentials are the most important principles, ideas, or facts of a particular subject.

    e.g. ...the essentials of everyday life, such as eating and exercise...
           饮食、锻炼等日常生活的要素
    e.g. This has stripped the contest down to its essentials.
           这就使比赛精简到最基本的环节。