The Collaborative International Dictionary of English v.0.48:

AEcidium \[AE]*cid"i*um\, n.; pl. [AE]cidia. [NL., dim. of Gr.
   ? injury.] (Bot.)
   A form of fruit in the cycle of development of the Rusts or
   Brands, an order of fungi, formerly considered independent
   plants.
   [1913 Webster]