The Collaborative International Dictionary of English v.0.48:

Sprat \Sprat\ (spr[a^]t), n. [OE. sprot, sprotte, D. sprot; akin
   to G. sprotte.] (Zool.)
   (a) A small European herring (Clupea sprattus) closely
       allied to the common herring and the pilchard; -- called
       also garvie. The name is also applied to small herring
       of different kinds.
   (b) A California surf-fish (Rhacochilus toxotes); -- called
       also alfione, and perch.
       [1913 Webster]

   Sprat borer (Zool.), the red-throated diver; -- so called
      from its fondness for sprats. See Diver.

   Sprat loon. (Zool.)
   (a) The young of the great northern diver. [Prov. Eng.]
   (b) The red-throated diver. See Diver.

   Sprat mew (Zool.), the kittiwake gull.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sprat
    n 1: small fatty European fish; usually smoked or canned like
         sardines [syn: sprat, brisling]
    2: small herring processed like a sardine [syn: brisling,
       sprat, Clupea sprattus]