essential ; indispensable
必不可缺的条件 conditio sine qua non
生存的必不可缺的 sine qua non
视镜是必不可缺的，可是它们妨碍了视线。
Glasses are, of course, an indispensable thing, but they worsen the sight.
youdao
空气、食物和水是维持生命必不可缺的。
Air, food and water are necessary to support life.
关注——什么是我们的生活中至关重要和必不可缺的。
Focus - on what is important and essential to our lives.
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