Deflagration

A subsonic combustion that usually propagates through thermal conductivity (hot burning material heats the next layer of cold material and ignites it)
Reference: Gexcon Handbook http://www.gexcon.com/handbook/GEXHBchap2.htm
Explosion propagating at subsonic velocity.
Reference: S2S (safety to safety website: ISO 8421-1(1987-03-01, 1.11), EN 1127-1: 1998, EN 1127-2:2002 (E), prEN 14460:2002 (E), EN 13237:2003 (E))
A propagating exothermic decomposition zone which generates hot gases (flame) resulting in pressure build-up. The propagation speed can vary over a wide range depending on substance structure, its phase, and feed-back processes from pressure increase and turbulence causing flame acceleration but the velocity is always subsonic relative to the local sound velocity.
Science/Flammability/Explosions/Phenomena