indispensable

释义：“不可或缺的”，是一个很常见的形容事物必要性的词，往往后接part, role, component, element, ingredient等等。

例句：
A person's skin was an indispensable clue as to one's true age.
Antimicrobial drugs have played an indispensable role in decreasing illness and death associated with infectious diseases in animals and humans.