Source : Webster's Revised Unabridged Dictionary (1913)

Topmast \Top"mast\, n. (Naut.)
   The second mast, or that which is next above the lower mast,
   and below the topgallant mast.

Source : WordNet®

topmast
     n : the mast next above a lower mast and topmost in a
         fore-and-aft rig