main or chief or principal support, anchor (to windward), sheet anchor, bulwark, buttress, linchpin, main or greatest strength:
David remains the mainstay of the sales force.
Advanced English Dictionary
+noun [usually sing.] ~ (of sth) a person or thing that is the most important part of sth and enables it to exist or be successful: He is the mainstay of our theatre group. + Cocoa is the country's economic mainstay. + Love songs form the mainstay of his act.
Random quote: Small opportunities are often the beginning of great enterprises.: Demosthenes