mainstay
/ˈmeɪnsteɪ ˈमेन्स्टे/ noun [C] a person or thing that is the most important part of something, which makes it possible for it to exist or to be successful किसी वस्तु, गतिविधि आदि के अस्तित्व या सफलता का मुख्य आधार (व्यक्ति या वस्तु)
Agriculture is the mainstay of the country’s economy.