The Collaborative International Dictionary of English v.0.48:

Gall \Gall\ (g[add]l), n. [F. galle, noix de galle, fr. L.
   galla.] (Zool.)
   An excrescence of any form produced on any part of a plant by
   insects or their larvae. They are most commonly caused by
   small Hymenoptera and Diptera which puncture the bark and lay
   their eggs in the wounds. The larvae live within the galls.
   Some galls are due to aphids, mites, etc. See Gallnut.
   [1913 Webster]

   Note: The galls, or gallnuts, of commerce are produced by
         insects of the genus Cynips, chiefly on an oak
         (Quercus infectoria syn. Quercus Lusitanica) of
         Western Asia and Southern Europe. They contain much
         tannin, and are used in the manufacture of that article
         and for making ink and a black dye, as well as in
         medicine.
         [1913 Webster]

   Gall insect (Zool.), any insect that produces galls.

   Gall midge (Zool.), any small dipterous insect that
      produces galls.

   Gall oak, the oak (Quercus infectoria) which yields the
      galls of commerce.

   Gall of glass, the neutral salt skimmed off from the
      surface of melted crown glass;- called also glass gall
      and sandiver. --Ure.

   Gall wasp. (Zool.) See Gallfly.
      [1913 Webster]

WordNet (r) 3.0 (2006):

gall midge
    n 1: fragile mosquito-like flies that produce galls on plants
         [syn: gall midge, gallfly, gall gnat]