Source : Webster's Revised Unabridged Dictionary (1913)

Mackerel \Mack`er*el\, n. [OF. maquerel, F. maquereau (LL.
   macarellus), prob. for maclereau, fr. L. macula a spot, in
   allusion to the markings on the fish. See {Mail} armor.]
   (Zo["o]l.)
   Any species of the genus {Scomber}, and of several related
   genera. They are finely formed and very active oceanic
   fishes. Most of them are highly prized for food.

   Note: The common mackerel ({Scomber scombrus}), which
         inhabits both sides of the North Atlantic, is one of
         the most important food fishes. It is mottled with
         green and blue. The Spanish mackerel ({Scomberomorus
         maculatus}), of the American coast, is covered with
         bright yellow circular spots.

   {Bull mackerel}, {Chub mackerel}. (Zo["o]l.) See under
      {Chub}.

   {Frigate mackerel}. See under {Frigate}.

   {Horse mackerel} . See under {Horse}.

   {Mackerel bird} (Zo["o]l.), the wryneck; -- so called because
      it arrives in England at the time when mackerel are in
      season.

   {Mackerel cock} (Zo["o]l.), the Manx shearwater; -- so called
      because it precedes the appearance of the mackerel on the
      east coast of Ireland.

   {Mackerel guide}. (Zo["o]l.) See {Garfish}
   (a) .

   {Mackerel gull} (Zo["o]l.) any one of several species of gull
      which feed upon or follow mackerel, as the kittiwake.

   {Mackerel midge} (Zo["o]l.), a very small oceanic gadoid fish
      of the North Atlantic. It is about an inch and a half long
      and has four barbels on the upper jaw. It is now
      considered the young of the genus {Onos}, or {Motella}.

   {Mackerel plow}, an instrument for creasing the sides of lean
      mackerel to improve their appearance. --Knight.

   {Mackerel shark} (Zo["o]l.), the porbeagle.

   {Mackerel sky}, or {Mackerel-back sky}, a sky flecked with
      small white clouds; a cirro-cumulus. See {Cloud}.

            Mackerel sky and mare's-tails Make tall ships carry
            low sails.                            --Old Rhyme.

Source : WordNet®

horse mackerel
     n 1: largest tuna; to 1500 pounds; of mostly temperate seas: feed
          in polar regions but breed in tropics [syn: {bluefin}, {bluefin
          tuna}, {Thunnus thynnus}]
     2: large elongated compressed food fish of the Atlantic waters
        of Europe [syn: {saurel}, {Trachurus trachurus}]
     3: a California food fish [syn: {jack mackerel}, {Spanish
        mackerel}, {saurel}, {Trachurus symmetricus}]