Outburst has been defined as a spontaneous ejection of gas and coal from the solid face, where the gas is a mixture of methane and carbon dioxide.

This definition is questioned in the use of the term "spontaneous" as this suggests an outburst occurs without triggering, when an outburst can be provoked with the use of explosives or during the cutting of coal. Thus, outbursts have also been defined as a phenomenon characterised by ejection from the solid face into the mine as a mixture of broken rock and gas.

Outbursts are known to originate in the upper section of the working face and may result in a cavity in and around the face. Outbursts usually occur from fresh, gassy, high rank coal during mining, especially during drivage and often but not always in the vicinity of geologic disturbances, faults and dykes. In some cases these outbursts include large volumes of a few tonnes of rock and a few hundred thousand cubic metres of gas.

The gases involved in outburst are generally methane and carbon dioxide, but there are also cases of nitrogen gas being the predominant outburst fuel. These gases when ejected not only cause injury to workers and damage to equipment around the working face, but also cause the mine atmosphere to become toxic for the workers to breathe and can create an explosive gas combination.

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