Definition of mainstay noun from the Oxford Advanced American Dictionary

 

mainstay

 noun
noun
NAmE//ˈmeɪnsteɪ//
 
[usually singular]mainstay (of something)
 
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a person or thing that is the most important part of something and enables it to exist or be successful He is the mainstay of our theater group. Cocoa is the country's economic mainstay. Love songs form the mainstay of his act.
See the Oxford Advanced Learner's Dictionary entry: mainstay