The Collaborative International Dictionary of English v.0.48:

matzo \matz"o\, matzoh \matz"oh\, n.; pl. E. matzos or
   matzohs; Hebr. matzoth [Heb. matsts[=o]th, pl. of
   matsts[=a]h unleavened.]
   A cake of unleavened bread eaten by the Jews at the feast of
   the Passover.
   [1913 Webster]

WordNet (r) 3.0 (2006):

matzoh
    n 1: brittle flat bread eaten at Passover [syn: matzo,
         matzoh, matzah, unleavened bread]