Oxidation
Definition
Losing electrons. Remember OIL: Oxidation Is Loss. When iron rusts, iron atoms are oxidized.
Technical definition
Oxidation is the loss of electrons by a species during a redox reaction, corresponding to an increase in oxidation state. The oxidized species is the reducing agent, and oxidation always accompanies reduction.
Example
In Zn + Cu^2+ -> Zn^2+ + Cu, zinc is oxidized: each Zn atom loses 2 electrons to become Zn^2+.
Common confusion
Oxidation does not require oxygen; the term is historical. What matters is electron loss, so the reaction of sodium with chlorine involves oxidation of sodium with no oxygen anywhere.