The Collaborative International Dictionary of English v.0.48:
Topmast \Top"mast\, n. (Naut.) The second mast, or that which is next above the lower mast, and below the topgallant mast. [1913 Webster]WordNet (r) 3.0 (2006):
topmast n 1: the mast next above a lower mast and topmost in a fore-and- aft rig