The two parts of an oxidation-reduction reaction, one representing oxidation, the other reduction.
Ag+ + Cu → Ag + Cu2+
the half reactions are:
Ag+ → Ag
Cu → Cu2+
The silver is being reduced, its oxidation number going from +1 to zero.
The copper′s oxidation number went from zero to +2 and so it was oxidized in the reaction.
See also: Chemical Reaction.