The Collaborative International Dictionary of English v.0.48:

Hardtack \Hard"tack`\ or Hard-tack
\Hard"-tack`\(h[aum]rd"t[a^]k`), n.
   1. A name given by soldiers and sailors to a kind of
      unleavened hard biscuit or sea bread. Called also pilot
      biscuit, pilot bread, ship biscuit and ship bread
      [1913 Webster]

   2. Any of several mahogany trees, esp. the Cercocarpus
      betuloides. --MW10
      [PJC]

WordNet (r) 3.0 (2006):

pilot biscuit
    n 1: very hard unsalted biscuit or bread; a former ship's staple
         [syn: hardtack, pilot biscuit, pilot bread, sea
         biscuit, ship biscuit]