A synthesis reaction is created in chemistry by the combination of two or more elements producing a compound. The two elements, or reactants, provide the basis of the chemical reaction. The formation of the compound is the 'synthesis' element, synthesizing the new material as a different material from the reactants. The term is used to identify the specific processes and elements of synthesis, as a discreet stage of the chemical functions.

Sodium and Chlorine combine to form salt.
Iron, when exposed to oxygen, forms iron oxide, or rust.
Alloys: copper and tin combine to form bronze as their compound.
http://www.ric.edu/faculty/ptiskus/reactions/Index.htm
http://en.wikipedia.org/wiki/Chemical_synthesis