Wiktionary
n. (context chemistry English) Any substance that oxidizes, or receives electrons from, another; in so doing, it becomes reduced.
WordNet
Wikipedia
In chemistry, an oxidizing agent is a substance that has the ability to oxidize other substances (cause them to lose electrons). Common oxidizing agents are oxygen, hydrogen peroxide and fluorine.
In one sense, an oxidizing agent is a chemical species that removes an electron from another species. It is one component in an oxidation–reduction (redox) reaction. In the second sense, an oxidizing agent or an oxidizer is a chemical species that transfers electronegative atoms, usually oxygen, to a substrate. Combustion, many explosives, and organic redox reactions involve atom-transfer reactions.