Source : Webster's Revised Unabridged Dictionary (1913)

Lead \Lead\ (l[e^]d), n. [OE. led, leed, lead, AS. le['a]d; akin
   to D. lood, MHG. l[=o]t, G. loth plummet, sounding lead,
   small weight, Sw. & Dan. lod. [root]123]
   1. (Chem.) One of the elements, a heavy, pliable, inelastic
      metal, having a bright, bluish color, but easily
      tarnished. It is both malleable and ductile, though with
      little tenacity, and is used for tubes, sheets, bullets,
      etc. Its specific gravity is 11.37. It is easily fusible,
      forms alloys with other metals, and is an ingredient of
      solder and type metal. Atomic weight, 206.4. Symbol Pb (L.
      Plumbum). It is chiefly obtained from the mineral galena,
      lead sulphide.

   2. An article made of lead or an alloy of lead; as:
      (a) A plummet or mass of lead, used in sounding at sea.
      (b) (Print.) A thin strip of type metal, used to separate
          lines of type in printing.
      (c) Sheets or plates of lead used as a covering for roofs;
          hence, pl., a roof covered with lead sheets or terne
          plates.

                I would have the tower two stories, and goodly
                leads upon the top.               --Bacon

   3. A small cylinder of black lead or plumbago, used in
      pencils.

   {Black lead}, graphite or plumbago; -- so called from its
      leadlike appearance and streak. [Colloq.]

   {Coasting lead}, a sounding lead intermediate in weight
      between a hand lead and deep-sea lead.

   {Deep-sea lead}, the heaviest of sounding leads, used in
      water exceeding a hundred fathoms in depth. --Ham. Nav.
      Encyc.

   {Hand lead}, a small lead use for sounding in shallow water.
      

   {Krems lead}, {Kremnitz lead} [so called from Krems or
      Kremnitz, in Austria], a pure variety of white lead,
      formed into tablets, and called also {Krems, or Kremnitz,
      white}, and {Vienna white}.

   {Lead arming}, tallow put in the hollow of a sounding lead.
      See {To arm the lead} (below).

   {Lead colic}. See under {Colic}.

   {Lead color}, a deep bluish gray color, like tarnished lead.
      

   {Lead glance}. (Min.) Same as {Galena}.

   {Lead line}
      (a) (Med.) A dark line along the gums produced by a
          deposit of metallic lead, due to lead poisoning.
      (b) (Naut.) A sounding line.

   {Lead mill}, a leaden polishing wheel, used by lapidaries.

   {Lead ocher} (Min.), a massive sulphur-yellow oxide of lead.
      Same as {Massicot}.

   {Lead pencil}, a pencil of which the marking material is
      graphite (black lead).

   {Lead plant} (Bot.), a low leguminous plant, genus {Amorpha}
      ({A. canescens}), found in the Northwestern United States,
      where its presence is supposed to indicate lead ore.
      --Gray.

   {Lead tree}.
      (a) (Bot.) A West Indian name for the tropical, leguminous
          tree, {Leuc[ae]na glauca}; -- probably so called from
          the glaucous color of the foliage.
      (b) (Chem.) Lead crystallized in arborescent forms from a
          solution of some lead salt, as by suspending a strip
          of zinc in lead acetate.

   {Mock lead}, a miner's term for blende.

   {Red lead}, a scarlet, crystalline, granular powder,
      consisting of minium when pure, but commonly containing
      several of the oxides of lead. It is used as a paint or
      cement and also as an ingredient of flint glass.

   {Red lead ore} (Min.), crocoite.

   {Sugar of lead}, acetate of lead.

   {To arm the lead}, to fill the hollow in the bottom of a
      sounding lead with tallow in order to discover the nature
      of the bottom by the substances adhering. --Ham. Nav.
      Encyc.

   {To} {cast, or heave}, {the lead}, to cast the sounding lead
      for ascertaining the depth of water.

   {White lead}, hydrated carbonate of lead, obtained as a
      white, amorphous powder, and much used as an ingredient of
      white paint.

Black lead \Black` lead"\
   Plumbago; graphite. It leaves a blackish mark somewhat like
   lead. See {Graphite}.

Source : WordNet®

black lead
     n : used as a lubricant and as a moderator in nuclear reactors
         [syn: {graphite}, {plumbago}]