Oxidation is the loss of electrons. When a species loses electrons it is said to be oxidised.
Na → Na+ + e– (each sodium atom loses one electron)
2I – → I2 + 2e– (each iodide ion loses one electron, so two in total)
Reduction is the gain of electrons. When a species gains electrons it is said to be reduced.
Cl2 + 2e– → 2Cl– (each chlorine atom gains one electron, so two in total)
Al3+ + 3e– → Al (each aluminum ion gains three electrons)
Processes which show the gain or loss of electrons by a species are known as half-equations. They show simple oxidation or reduction processes.