The Collaborative International Dictionary of English v.0.48:

Ascus \As"cus\, n.; pl. Asci. [NL., fr. Gr. ? a bladder.] (Bot.)
   A small membranous bladder or tube in which are inclosed the
   seedlike reproductive particles or sporules of lichens and
   certain fungi.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ascus
    n 1: saclike structure in which ascospores are formed through
         sexual reproduction of ascomycetes