Source : Webster's Revised Unabridged Dictionary (1913)

Peridium \Pe*rid"i*um\, n.; pl. {Peridia}. [NL., fr. Gr. ? about
   + ?, a dim. ending.] (Bot.)
   The envelope or coat of certain fungi, such as the puffballs
   and earthstars.

Source : WordNet®

peridium
     n : outer layer of the spore-bearing organ in many fungi
     [also: {peridia} (pl)]