The Collaborative International Dictionary of English v.0.48:

cortina \cor*ti"na\ (k[^o]r*t[imac]"n[.a] or
   k[^o]r*t[=e]"n[.a]), n.; pl. cortinae (k[^o]r*t[=e]"n[=e]).
   (Biology)
   a cobwebby remnant of the partial veil which in some mature
   mushrooms hang from the edges of the cap.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

cortina
    n 1: a cobwebby partial veil consisting of silky fibrils