The Collaborative International Dictionary of English v.0.48:

Zwieback \Zwie"back`\ (tsv[=e]"b[aum]k`), n. [G., fr. zwie- two,
   twice (see Twice) + backen to bake.]
   A kind of biscuit or rusk first baked in a loaf and
   afterwards cut and toasted.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

zwieback
    n 1: slice of sweet raised bread baked again until it is brown
         and hard and crisp [syn: zwieback, rusk, Brussels
         biscuit, twice-baked bread]