Source : Webster's Revised Unabridged Dictionary (1913)

Slate \Slate\, n. [OE. slat, OF. esclat a shiver, splinter, F.
   ['e]clat, fr. OF. esclater to shiver, to chip, F. ['e]clater,
   fr. OHG. sliezen to tear, slit, split, fr. sl[=i]zan to slit,
   G. schleissen. See {Slit}, v. t., and cf. {Eclat}.]
   1. (Min.) An argillaceous rock which readily splits into thin
      plates; argillite; argillaceous schist.

   2. Any rock or stone having a slaty structure.

   3. A prepared piece of such stone. Especially:
      (a) A thin, flat piece, for roofing or covering houses,
          etc.
      (b) A tablet for writing upon.

   4. An artificial material, resembling slate, and used for the
      above purposes.

   5. A thin plate of any material; a flake. [Obs.]

   6. (Politics) A list of candidates, prepared for nomination
      or for election; a list of candidates, or a programme of
      action, devised beforehand. [Cant, U.S.] --Bartlett.

   {Adhesive slate} (Min.), a kind of slate of a greenish gray
      color, which absorbs water rapidly, and adheres to the
      tongue; whence the name.

   {Aluminous slate}, or {Alum slate} (Min.), a kind of slate
      containing sulphate of alumina, -- used in the manufacture
      of alum.

   {Bituminous slate} (Min.), a soft species of sectile clay
      slate, impregnated with bitumen.

   {Hornblende slate} (Min.), a slaty rock, consisting
      essentially of hornblende and feldspar, useful for
      flagging on account of its toughness.

   {Slate ax} or {axe}, a mattock with an ax end, used in
      shaping slates for roofs, and making holes in them for the
      nails.

   {Slate clay} (Geol.), an indurated clay, forming one of the
      alternating beds of the coal measures, consisting of an
      infusible compound of alumina and silica, and often used
      for making fire bricks. --Tomlinson.

   {Slate globe}, a globe the surface of which is made of an
      artificial slatelike material.

   {Slate pencil}, a pencil of slate, or of soapstone, used for
      writing on a slate.

   {Slate rocks} (Min.), rocks which split into thin lamin[ae],
      not necessarily parallel to the stratification; foliated
      rocks.

   {Slate spar} (Min.), a variety of calcite of silvery white
      luster and of a slaty structure.

   {Transparent slate}, a plate of translucent material, as
      ground glass, upon which a copy of a picture, placed
      beneath it, can be made by tracing.

Source : WordNet®

slate pencil
     n : a pencil of soft slate (or soapstone) used for writing on a
         slate