The Collaborative International Dictionary of English v.0.48:

Mainmast \Main"mast`\, n. (Naut.)
   The principal mast in a ship or other vessel.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Mast \Mast\, n. [AS. maest, masc.; akin to D., G., Dan., & Sw.
   mast, Icel. mastr, and perh. to L. malus.]
   [1913 Webster]
   1. (Naut.) A pole, or long, strong, round piece of timber, or
      spar, set upright in a boat or vessel, to sustain the
      sails, yards, rigging, etc. A mast may also consist of
      several pieces of timber united by iron bands, or of a
      hollow pillar of iron or steel.
      [1913 Webster]

            The tallest pine
            Hewn on Norwegian hills, to be the mast
            Of some great ammiral.                --Milton.
      [1913 Webster]

   Note: The most common general names of masts are foremast,
         mainmast, and mizzenmast, each of which may be made
         of separate spars.
         [1913 Webster]

   2. (Mach.) The vertical post of a derrick or crane.
      [1913 Webster]

   3. (Aeronautics) A spar or strut to which tie wires or guys
      are attached for stiffening purposes.
      [Webster 1913 Suppl.]

   Afore the mast, Before the mast. See under Afore, and
      Before.

   Mast coat. See under Coat.

   Mast hoop, one of a number of hoops attached to the fore
      edge of a boom sail, which slip on the mast as the sail is
      raised or lowered; also, one of the iron hoops used in
      making a made mast. See Made.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mainmast
    n 1: the chief mast of a sailing vessel with two or more masts