Original: Solder (or or American) is a fusible metal alloy with a melting point or melting range of 90 t, used in a process called soldering where it is melted to join metallic surfaces.
Simplified: Solder is a metal or alloy that melts at a low temperature. There are two types of solder; soft solder (easily melting alloys that are melted with soldering irons and used for electronics and electrical work), and hard solder (high melting alloys or metals that are melted with a torch).