From Longman Dictionary of Contemporary Englishmainstaymain‧stay /ˈmeɪnsteɪ/ noun → the mainstay of something
Examples from the Corpus
mainstay• But more than being a mainstay of fashion, the T-shirt is an enduring medium of advertising.• Another mainstay of the opera company is Tatiana Troyanos.• Tourism has long been the economic mainstay of Kashmir.• For one thing, he came out and said he hated funk, a Peppers mainstay.• Milk products typical of nomadic pastoralists formed the mainstay of their diet.• Middle class women, once the mainstay of all volunteer endeavors, are no longer an unlimited resource.• The mainstay of treatment is a twice daily form of physiotherapy, usually done by the child's parents.From Longman Business Dictionarymainstaymain‧stay /ˈmeɪnsteɪ/ noun [countable usually singular] the most important part of something, that makes it possible for it to be successful or to continue to existmainstay ofCod fishing was the mainstay of the Newfoundland economy, accounting for 21% of all jobs.Polythene and PVC had been two of ICI’s mainstay products (=most important products). 