Source : Webster's Revised Unabridged Dictionary (1913)

Mainstay \Main"stay`\, n.
   1. (Naut.) The stay extending from the foot of the foremast
      to the maintop.

   2. Main support; principal dependence.

            The great mainstay of the Church.     --Buckle.

Source : WordNet®

mainstay
     n 1: a prominent supporter; "he is a pillar of the community"
          [syn: {pillar}]
     2: a central cohesive source of support and stability; "faith
        is his anchor"; "the keystone of campaign reform was the
        ban on soft money"; "he is the linchpin of this firm"
        [syn: {anchor}, {keystone}, {backbone}, {linchpin}, {lynchpin}]
     3: the forestay that braces the mainmast