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main‧stay

the mainstay of something
a) an important part of something that makes it possible for it to work properly or continue to exist :
Agriculture is still the mainstay of the country's economy.
Agriculture is still the mainstay of the country's economy.b) someone who does most of the important work for a group or organization :
She was the mainstay of the team.
She was the mainstay of the team.