Deflagration to detonation transition

Deflagration to detonation transition (DDT) refers to a phenomenon in which a combustion process transitions from deflagration (subsonic flame propagation) to a detonation (supersonic shock-coupled combustion). In a deflagration, the flame front grows through heat transfer and diffusion at speeds that are below the speed of sound, while in contrary in detonation, a shock wave compresses on the unreacted mixture and ignites, causing the reaction to grow at supersonic speeds.

Source: Wikipedia — Deflagration to detonation transition (CC BY-SA 4.0)