What is the meaning of mainstay?

noun

• A prominent supporter.
  • usage: "he is a pillar of the community"
  • synonyms: pillar
• A central cohesive source of support and stability.
  • usage: "faith is his anchor"; "the keystone of campaign reform was the ban on soft money"; "he is the linchpin of this firm"
  • synonyms: anchor, keystone, backbone, linchpin, lynchpin
• The forestay that braces the mainmast. .