Source : Webster's Revised Unabridged Dictionary (1913)

Solder \Sol"der\, n. [Formerly soder; F. soudure, OF. soudeure,
   fr. OF. & F. souder to solder, L. solidare to fasten, to make
   solid. See {Solid}, and cf. {Sawder}.]
   A metal or metallic alloy used when melted for uniting
   adjacent metallic edges or surfaces; a metallic cement.
   Hence, anything which unites or cements.

   {Hard solder}, a solder which fuses only at a red heat, as
      one composed of zinc and copper, or silver and copper,
      etc.

   {Soft solder}, a solder fusible at comparatively low
      temperatures; as, plumbers' solder, consisting of two
      parts lead and one part tin, is a soft solder.

Source : WordNet®

soft solder
     n : solder that melts at a relatively low temperature