basic 普通用词，指明确、具体的基础或起点。
essential 语气比basic和fundamental强，强调必不可少，暗含某物如缺少某部分，则失去本质特征的意味。
fundamental 书面用词，不如basic使用广泛，侧重指作为基础、根本的抽象的事物。
radical 着重指事物的根本或其来源。
vital 正式用词，侧重指维持生命、活力必不可少的，或生死攸关的。
参考例句
1. It is an essential aspect of forgiveness. 这是宽恕中的一个必要方面。
2. Time keeping is essential to dream weaving. 时间一致是梦想编织的要素。
3. Copy control is an essential part of defining any C++ class. 复制控制是定义任意C++类必不可少的部分。
英英解释
n.
1. anything indispensable
2. absolutely necessary; vitally necessary
3. basic and fundamental
4. of the greatest importance
5. being or relating to or containing the essence of a plant etc
6. defining rights and duties as opposed to giving the rules by which rights and duties are established
7. absolutely required and not to be used up or sacrificed
