'Mainmast' definitions:

Definition of 'mainmast'

noun

The chief mast of a sailing vessel with two or more masts

From: GCIDE

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

From: GCIDE

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]