The Collaborative International Dictionary of English v.0.48:

Shad \Shad\ (sh[a^]d), n. sing. & pl. [AS. sceadda a kind of
   fish, akin to Prov. G. schade; cf. Ir. & Gael. sgadan a
   herring, W. ysgadan herrings; all perhaps akin to E. skate a
   fish.] (Zool.)
   Any one of several species of food fishes of the Herring
   family. The American species (Alosa sapidissima formerly
   Clupea sapidissima), which is abundant on the Atlantic
   coast and ascends the larger rivers in spring to spawn, is an
   important market fish. The European allice shad, or alose
   (Alosa alosa formerly Clupea alosa), and the twaite shad
   (Alosa finta formerly Clupea finta), are less important
   species. [Written also chad.]
   [1913 Webster]

   Note: The name is loosely applied, also, to several other
         fishes, as the gizzard shad (see under Gizzard),
         called also mud shad, white-eyed shad, and winter
         shad.
         [1913 Webster]

   Hardboaded shad, or Yellow-tailed shad, the menhaden.

   Hickory shad, or Tailor shad, the mattowacca.

   Long-boned shad, one of several species of important food
      fishes of the Bermudas and the West Indies, of the genus
      Gerres.

   Shad bush (Bot.), a name given to the North American shrubs
      or small trees of the rosaceous genus Amelanchier
      (Amelanchier Canadensis, and Amelanchier alnifolia).
      Their white racemose blossoms open in April or May, when
      the shad appear, and the edible berries (pomes) ripen in
      June or July, whence they are called Juneberries. The
      plant is also called service tree, and Juneberry.

   Shad frog, an American spotted frog (Rana halecina); --
      so called because it usually appears at the time when the
      shad begin to run in the rivers.

   Trout shad, the squeteague.

   White shad, the common shad.
      [1913 Webster]