a) IMPORTANTan important part of something that makes it possible for it to work properly or continue to exist …的主要依靠[支柱]
Agriculture is still the mainstay of the country’s economy.
农业仍然是这个国家经济的支柱。
b) MAINsomeone who does most of the important work for a group or organization …中挑大梁的人物[台柱子，骨干]
She was the mainstay of the team.
她是这个团队的骨干。
Examples from the Corpus
the mainstay of something• Inflections, positions, and signpost words are the mainstay of syntax.• What we once regarded as the mainstay of our society has become the exception and not the rule.• Milk products typical of nomadic pastoralists formed the mainstay of their diet.• Newsome is the mainstay of our central defence and worth all your Ibrox millions and more.• In terms of the size of the contribution, then, it is the informal sector which is the mainstay of community care.• Middle class women, once the mainstay of all volunteer endeavors, are no longer an unlimited resource.• Agriculture was the mainstay of the community, although most other rural crafts had disappeared.• Cloth - mainly wool and linen - was the mainstay of this trade.
→ the mainstay of something at mainstay(1)