Oxidizing Agent
Definition
The substance that causes oxidation by taking electrons, and gets reduced itself in the process.
Technical definition
An oxidizing agent (oxidant) is the electron acceptor in a redox reaction. It oxidizes another species while being reduced itself, and stronger oxidizing agents have more positive standard reduction potentials.
Example
In the thermite reaction, iron(III) oxide is the oxidizing agent: it takes electrons from aluminum, becoming iron metal.
Common confusion
The oxidizing agent gets REDUCED. The names describe what the agent does to others, not what happens to it: this is the most common redox mix-up.