The Collaborative International Dictionary of English v.0.48:

Endospore \En"do*spore\, n. [Endo- + spore.] (Bot.)
   The thin inner coat of certain spores.
   [1913 Webster]

WordNet (r) 3.0 (2006):

endospore
    n 1: a small asexual spore that develops inside the cell of some
         bacteria and algae