integral

adj. /ˈɪntɪɡrəl/ C1

不可或缺的；构成整体所必需的

Necessary to make a whole complete; essential.

仙境例句

The hookah was an integral part of the Caterpillar; nobody ever saw it without one.

水烟袋是毛毛虫不可缺少的一部分，谁也没见过它不拿着水烟袋。