The Collaborative International Dictionary of English v.0.48:

Zygospore \Zyg"o*spore\, n. [Gr. zygo`n a yoke + E. spore.]
   (Bot.)
   (a) Same as Zygosperm.
   (b) A spore formed by the union of several zoospores; --
       called also zygozoospore.
       [1913 Webster]

WordNet (r) 3.0 (2006):

zygospore
    n 1: a plant spore formed by two similar sexual cells