Reduction
Definition
Gaining electrons. Remember RIG: Reduction Is Gain. When copper ions become copper metal, they are reduced.
Technical definition
Reduction is the gain of electrons by a species during a redox reaction, corresponding to a decrease in oxidation state. The reduced species is the oxidizing agent, and reduction always accompanies oxidation.
Example
In Zn + Cu^2+ -> Zn^2+ + Cu, Cu^2+ is reduced: each copper ion gains 2 electrons to become copper metal.
Common confusion
Reduction reduces the oxidation NUMBER, not necessarily in an intuitive charge direction, and every reduction needs a matching oxidation: electrons cannot appear from nowhere.