Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

ascus
     n : saclike structure in which ascospores are formed through
         sexual reproduction of ascomycetes
     [also: {asci} (pl)]