mainstay
definition
a primary support or essential element
example
Agriculture has always been the mainstay of the local economy, providing jobs and resources for the community.
synonyms
pillarbackbonecornerstone
word origin — The word 'mainstay' originates from the late Middle English 'main stai,' where 'main' means 'chief' or 'primary' and 'stay' refers to 'support.'