The Collaborative International Dictionary of English v.0.48:

Iron \I"ron\ ([imac]"[u^]rn), n. [OE. iren, AS. [imac]ren,
   [imac]sen, [imac]sern; akin to D. ijzer, OS. [imac]sarn, OHG.
   [imac]sarn, [imac]san, G. eisen, Icel. [imac]sarn, j[=a]rn,
   Sw. & Dan. jern, and perh. to E. ice; cf. Ir. iarann, W.
   haiarn, Armor. houarn.]
   [1913 Webster]
   1. (Chem.) The most common and most useful metallic element,
      being of almost universal occurrence, usually in the form
      of an oxide (as hematite, magnetite, etc.), or a hydrous
      oxide (as limonite, turgite, etc.). It is reduced on an
      enormous scale in three principal forms; viz., cast
      iron, steel, and wrought iron. Iron usually appears
      dark brown, from oxidation or impurity, but when pure, or
      on a fresh surface, is a gray or white metal. It is easily
      oxidized (rusted) by moisture, and is attacked by many
      corrosive agents. Symbol Fe (Latin Ferrum). Atomic number
      26, atomic weight 55.847. Specific gravity, pure iron,
      7.86; cast iron, 7.1. In magnetic properties, it is
      superior to all other substances.
      [1913 Webster]

   Note: The value of iron is largely due to the facility with
         which it can be worked. Thus, when heated it is
         malleable and ductile, and can be easily welded and
         forged at a high temperature. As cast iron, it is
         easily fusible; as steel, is very tough, and (when
         tempered) very hard and elastic. Chemically, iron is
         grouped with cobalt and nickel. Steel is a variety of
         iron containing more carbon than wrought iron, but less
         that cast iron. It is made either from wrought iron, by
         roasting in a packing of carbon (cementation) or from
         cast iron, by burning off the impurities in a Bessemer
         converter (then called Bessemer steel), or directly
         from the iron ore (as in the Siemens rotatory and
         generating furnace).
         [1913 Webster]

   2. An instrument or utensil made of iron; -- chiefly in
      composition; as, a flatiron, a smoothing iron, etc.
      [1913 Webster]

            My young soldier, put up your iron.   --Shak.
      [1913 Webster]

   3. pl. Fetters; chains; handcuffs; manacles.
      [1913 Webster]

            Four of the sufferers were left to rot in irons.
                                                  --Macaulay.
      [1913 Webster]

   4. Strength; power; firmness; inflexibility; as, to rule with
      a rod of iron.
      [1913 Webster]

   5. (Golf) An iron-headed club with a deep face, chiefly used
      in making approaches, lifting a ball over hazards, etc.
      [Webster 1913 Suppl.]

   Bar iron. See Wrought iron (below).

   Bog iron, bog ore; limonite. See Bog ore, under Bog.

   Cast iron (Metal.), an impure variety of iron, containing
      from three to six percent of carbon, part of which is
      united with a part of the iron, as a carbide, and the rest
      is uncombined, as graphite. It there is little free
      carbon, the product is white iron; if much of the carbon
      has separated as graphite, it is called gray iron. See
      also Cast iron, in the Vocabulary.

   Fire irons. See under Fire, n.

   Gray irons. See under Fire, n.

   Gray iron. See Cast iron (above).

   It irons (Naut.), said of a sailing vessel, when, in
      tacking, she comes up head to the wind and will not fill
      away on either tack.

   Magnetic iron. See Magnetite.

   Malleable iron (Metal.), iron sufficiently pure or soft to
      be capable of extension under the hammer; also, specif., a
      kind of iron produced by removing a portion of the carbon
      or other impurities from cast iron, rendering it less
      brittle, and to some extent malleable.

   Meteoric iron (Chem.), iron forming a large, and often the
      chief, ingredient of meteorites. It invariably contains a
      small amount of nickel and cobalt. Cf. Meteorite.

   Pig iron, the form in which cast iron is made at the blast
      furnace, being run into molds, called pigs.

   Reduced iron. See under Reduced.

   Specular iron. See Hematite.

   Too many irons in the fire, too many objects or tasks
      requiring the attention at once.

   White iron. See Cast iron (above).

   Wrought iron (Metal.), the purest form of iron commonly
      known in the arts, containing only about half of one per
      cent of carbon. It is made either directly from the ore,
      as in the Catalan forge or bloomery, or by purifying
      (puddling) cast iron in a reverberatory furnace or
      refinery. It is tough, malleable, and ductile. When formed
      into bars, it is called bar iron.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Cast iron \Cast" i`ron\
   Highly carbonized iron, the direct product of the blast
   furnace; -- used for making castings, and for conversion into
   wrought iron and steel. It can not be welded or forged, is
   brittle, and sometimes very hard. Besides carbon, it contains
   sulphur, phosphorus, silica, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

cast iron
    n 1: an alloy of iron containing so much carbon that it is
         brittle and so cannot be wrought but must be shaped by
         casting