An oxidizer or oxidizing agent is a chemical compound that readily transfers oxygen atoms or accepts electrons from other compounds.
An oxidizing agent does not need to have oxygen, but by defintion must take electrons from another chemical (the "reducer" or "fuel"). In Pyrotechnics we are interested in oxidation as a thermic reaction. Thermic reactions are chemical reactions which have transfers in heat enrgy: either exothermic (producing heat) or endothermic (absorbing heat).
The most common oxidizers used in pyrotechnics are, Potassium Nitrate, Potassium Chlorate, and Potassium PerChlorate.