What is "pilot biscuit"

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

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

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