[1] : The Collaborative International Dictionary of English v.0.48

soap \soap\, n. oe. sope, as. s=ape; akin to d. zeep, g.
   seife, ohg. seifa, icel. s=apa, sw. s?pa, dan. s?be, and
   perhaps to as. simacpan to drip, mhg. simacfen, and l.
   sebum tallow. cf. saponaceous.
   a substance which dissolves in water, thus forming a lather,
   and is used as a cleansing agent. soap is produced by
   combining fats or oils with alkalies or alkaline earths,
   usually by boiling, and consists of salts of sodium,
   potassium, etc., with the fatty acids oleic, stearic,
   palmitic, etc.. see the note below, and cf.
   saponification. by extension, any compound of similar
   composition or properties, whether used as a cleaning agent
   or not.
   1913 webster

   note: in general, soaps are of two classes, hard and soft.
         calcium, magnesium, lead, etc., form soaps, but they
         are insoluble and useless.
         1913 webster

               the purifying action of soap depends upon the
               fact that it is decomposed by a large quantity of
               water into free alkali and an insoluble acid
               salt. the first of these takes away the fatty
               dirt on washing, and the latter forms the soap
               lather which envelops the greasy matter and thus
               tends to remove it.                --roscoe 
                                                  schorlemmer.
         1913 webster

   castile soap, a fine-grained hard soap, white or mottled,
      made of olive oil and soda; -- called also marseilles
      soap or venetian soap.

   hard soap, any one of a great variety of soaps, of
      different ingredients and color, which are hard and
      compact. all solid soaps are of this class.

   lead soap, an insoluble, white, pliable soap made by
      saponifying an oil olive oil with lead oxide; -- used
      externally in medicine. called also lead plaster,
      diachylon, etc.

   marine soap. see under marine.

   pills of soap med., pills containing soap and opium.

   potash soap, any soap made with potash, esp. the soft
      soaps, and a hard soap made from potash and castor oil.

   pumice soap, any hard soap charged with a gritty powder, as
      silica, alumina, powdered pumice, etc., which assists
      mechanically in the removal of dirt.

   resin soap, a yellow soap containing resin, -- used in
      bleaching.

   silicated soap, a cheap soap containing water glass sodium
      silicate.

   soap bark. bot. see quillaia bark.

   soap bubble, a hollow iridescent globe, formed by blowing a
      film of soap suds from a pipe; figuratively, something
      attractive, but extremely unsubstantial.
      1913 webster

            this soap bubble of the metaphysicians. --j. c.
                                                  shairp.
      1913 webster

   soap cerate, a cerate formed of soap, olive oil, white wax,
      and the subacetate of lead, sometimes used as an
      application to allay inflammation.

   soap fat, the refuse fat of kitchens, slaughter houses,
      etc., used in making soap.

   soap liniment med., a liniment containing soap, camphor,
      and alcohol.

   soap nut, the hard kernel or seed of the fruit of the
      soapberry tree, -- used for making beads, buttons, etc.

   soap plant bot., one of several plants used in the place
      of soap, as the chlorogalum pomeridianum, a california
      plant, the bulb of which, when stripped of its husk and
      rubbed on wet clothes, makes a thick lather, and smells
      not unlike new brown soap. it is called also soap apple,
      soap bulb, and soap weed.

   soap tree. bot. same as soapberry tree.

   soda soap, a soap containing a sodium salt. the soda soaps
      are all hard soaps.

   soft soap, a soap of a gray or brownish yellow color, and
      of a slimy, jellylike consistence, made from potash or the
      lye from wood ashes. it is strongly alkaline and often
      contains glycerin, and is used in scouring wood, in
      cleansing linen, in dyehouses, etc. figuratively,
      flattery; wheedling; blarney. colloq.

   toilet soap, hard soap for the toilet, usually colored and
      perfumed.
      1913 webster
see also:
saponaceous saponification castile soap marseilles soap venetian soap hard soap 
lead soap lead plaster diachylon marine soap marine 
pills of soap potash soap pumice soap resin soap silicated soap 
soap bark quillaia bark soap bubble soap cerate soap fat 
soap liniment soap nut soap plant chlorogalum pomeridianum soap apple 
soap bulb soap weed soap tree soapberry tree soda soap 
soft soap toilet soap 

[2] : The Collaborative International Dictionary of English v.0.48

castile soap \cas"tile soap`\ from castile, or castilia, a
   province in spain, from which it originally came.
   a kind of fine, hard, white or mottled soap, made with olive
   oil and soda; also, a soap made in imitation of the
   above-described soap.
   1913 webster

[3] : WordNet (r) 2.0

castile soap
     n : a good hard soap made from olive oil and sodium hydroxide

Castile >> C Words
Castile, definition of term: Castile
castile_pag1.html

Soap >> S Words
Soap, definition of term: Soap
soap_pag1.html

Soap Opera >> S Words
Soap Opera, definition of term: Soap Opera
soap+opera_pag1.html

Soap Bark >> S Words
Soap Bark, definition of term: Soap Bark
soap+bark_pag1.html

Bar Soap >> B Words
Bar Soap, definition of term: Bar Soap
bar+soap_pag1.html