Either substances that promote combustion, or substances that spontaneously release oxygen at room temperature or upon slight warming. They include peroxides, chlorates, perchlorates, nitrates, and permanganates (clearly, not a chemist's definition)
University of Maine, Department of Chemistry, Glossary of Safety Terms and Acronyms, Orono, ME, USA 2009. http://www.business.govt.nz/worksafe/tools-resourc es/glossary-of-terms-and-acronyms/a-to-b, (ChemSfty - Amer) {503}