mainstay
/ˈmeɪnsteɪ/noun
1- a stay which extends from the maintop to the foot of the foremast of a sailing ship（帆船）主桅支索。
1.1
- figurative a thing on which something else is based or depends〈喻〉主要依靠; 主要资源; 支柱：
whitefish are the mainstay of the local industry.
白鲑是当地的支柱产业。