Reducing Agent
Definition
The substance that causes reduction by giving away electrons, and gets oxidized itself in the process.
Technical definition
A reducing agent (reductant) is the electron donor in a redox reaction. It reduces another species while being oxidized itself, and stronger reducing agents have more negative standard reduction potentials.
Example
Carbon monoxide in a blast furnace is the reducing agent: it donates electrons to iron ore, becoming CO2 while iron is freed as metal.
Common confusion
The reducing agent gets OXIDIZED. Again, the name describes its effect on the other species; the agent itself loses electrons.