The Collaborative International Dictionary of English v.0.48:

Soap \Soap\, n. [OE. sope, AS. s[=a]pe; akin to D. zeep, G.
   seife, OHG. seifa, Icel. s[=a]pa, Sw. s?pa, Dan. s?be, and
   perhaps to AS. s[imac]pan to drip, MHG. s[imac]fen, 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]

The Collaborative International Dictionary of English v.0.48:

Soapberry tree \Soap"ber`ry tree`\ (Bot.)
   Any tree of the genus Sapindus, esp. Sapindus saponaria,
   the fleshy part of whose fruit is used instead of soap in
   washing linen; -- also called soap tree.
   [1913 Webster]

WordNet (r) 3.0 (2006):

soap tree
    n 1: tall arborescent yucca of southwestern United States [syn:
         soapweed, soap-weed, soap tree, Yucca elata]