necessary的同義詞有：

indispensable，essential，requisite

這些形容詞均有“必不可少的，必需的”之意。

indispensable語氣強，側重指絕對必要性或不可缺少性。

Cars have become an indispensable part of our lives.

essential指對事物本身性質具有決定性的重要性，暗含不可缺少，否則便無法存在。

It was absolutely essential to separate crops from the areas that animals used as pasture.

necessary最常用詞，側重由客觀因素決定的某種需要，強調不可避免，但並非絕不可缺少。

Scientific work is differentiated from art by its necessary connection with the idea of progress.

requisite正式用詞，特指為實現某一目的或得到某種結果而應有的需要，有時可與necessary換用。

She lacks the requisite experience for the job.