Oxidation is the loss of electrons from an atom or ion. It can also be defined as an increase in the oxidation state of an element. Oxidation cannot occur without the opposite process, reduction, taking place at the same time. If electrons are lost from one species they must be gained by another. Copper looses 2 electrons to form copper 2+ ions, where the copper has been oxidised, as it has lost electrons.
An oxidising agent is a species that can bring about the oxidation of a substance. It does this by accepting the electrons lost. This results in the oxidising agent itself being reduced in the process. An example of this is the activated carrier molecule called NADH, this molecule carries high energy electrons and hydrogen atoms and it becomes reduced in the process of food molecule oxidation from NAD+..
All oxidation reactions also involve reduction and so the overall processes are referred to as a redox reaction.