Source : Webster's Revised Unabridged Dictionary (1913)

   {Antimony rubber}, an elastic durable variety of vulcanized
      caoutchouc of a red color. It contains antimony sulphide
      as an important constituent.

   {Hard rubber}, a kind of vulcanized caoutchouc which nearly
      resembles horn in texture, rigidity, etc.

   {India rubber}, caoutchouc. See {Caoutchouc}.

   {Rubber cloth}, cloth covered with caoutchouc for excluding
      water or moisture.

   {Rubber dam} (Dentistry), a shield of thin sheet rubber
      clasped around a tooth to exclude saliva from the tooth.

Hard \Hard\, a. [Compar. {Harder}; superl. {Hardest}.] [{OE}.
   heard, AS. heard; akin to OS. & D. heard, G. hart, OHG.
   harti, Icel. har?r, Dan. haard, Sw. h[*a]rd, Goth. hardus,
   Gr.? strong, ?, ?, strength, and also to E. -ard, as in
   coward, drunkard, -crat, -cracy in autocrat, democracy; cf.
   Skr. kratu strength, ? to do, make. Cf. {Hardy}.]
   1. Not easily penetrated, cut, or separated into parts; not
      yielding to pressure; firm; solid; compact; -- applied to
      material bodies, and opposed to soft; as, hard wood; hard
      flesh; a hard apple.

   2. Difficult, mentally or judicially; not easily apprehended,
      decided, or resolved; as a hard problem.

            The hard causes they brought unto Moses. --Ex.
                                                  xviii. 26.

            In which are some things hard to be understood. --2
                                                  Peter iii. 16.

   3. Difficult to accomplish; full of obstacles; laborious;
      fatiguing; arduous; as, a hard task; a disease hard to
      cure.

   4. Difficult to resist or control; powerful.

            The stag was too hard for the horse.  --L'Estrange.

            A power which will be always too hard for them.
                                                  --Addison.

   5. Difficult to bear or endure; not easy to put up with or
      consent to; hence, severe; rigorous; oppressive;
      distressing; unjust; grasping; as, a hard lot; hard times;
      hard fare; a hard winter; hard conditions or terms.

            I never could drive a hard bargain.   --Burke.

   6. Difficult to please or influence; stern; unyielding;
      obdurate; unsympathetic; unfeeling; cruel; as, a hard
      master; a hard heart; hard words; a hard character.

   7. Not easy or agreeable to the taste; stiff; rigid;
      ungraceful; repelling; as, a hard style.

            Figures harder than even the marble itself.
                                                  --Dryden.

   8. Rough; acid; sour, as liquors; as, hard cider.

   9. (Pron.) Abrupt or explosive in utterance; not aspirated,
      sibilated, or pronounced with a gradual change of the
      organs from one position to another; -- said of certain
      consonants, as c in came, and g in go, as distinguished
      from the same letters in center, general, etc.

   10. Wanting softness or smoothness of utterance; harsh; as, a
       hard tone.

   11. (Painting)
       (a) Rigid in the drawing or distribution of the figures;
           formal; lacking grace of composition.
       (b) Having disagreeable and abrupt contrasts in the
           coloring or light and shade.

   {Hard cancer}, {Hard case}, etc. See under {Cancer}, {Case},
      etc.

   {Hard clam}, or {Hard-shelled clam} (Zo["o]l.), the guahog.
      

   {Hard coal}, anthracite, as distinguished from bituminous or
      soft coal.

   {Hard and fast}. (Naut.) See under {Fast}.

   {Hard finish} (Arch.), a smooth finishing coat of hard fine
      plaster applied to the surface of rough plastering.

   {Hard lines}, hardship; difficult conditions.

   {Hard money}, coin or specie, as distinguished from paper
      money.

   {Hard oyster} (Zo["o]l.), the northern native oyster. [Local,
      U. S.]

   {Hard pan}, the hard stratum of earth lying beneath the soil;
      hence, figuratively, the firm, substantial, fundamental
      part or quality of anything; as, the hard pan of
      character, of a matter in dispute, etc. See {Pan}.

   {Hard rubber}. See under {Rubber}.

   {Hard solder}. See under {Solder}.

   {Hard water}, water, which contains lime or some mineral
      substance rendering it unfit for washing. See {Hardness},
      3.

   {Hard wood}, wood of a solid or hard texture; as walnut, oak,
      ash, box, and the like, in distinction from pine, poplar,
      hemlock, etc.

   {In hard condition}, in excellent condition for racing;
      having firm muscles;-said of race horses.

   Syn: Solid; arduous; powerful; trying; unyielding; stubborn;
        stern; flinty; unfeeling; harsh; difficult; severe;
        obdurate; rigid. See {Solid}, and {Arduous}.

Source : WordNet®

hard rubber
     n : a hard nonresilient rubber formed by vulcanizing natural
         rubber [syn: {vulcanite}, {ebonite}]