Is it true that explosion is a type of combustion in which the substance suddenly catches fire without the supply of heat or friction externally 

The given statement is false.

Explosion is a type of combustion during which a huge amount of heat and light is evolved with the production of gas, for example bursting of crackers. Combustion in which the substance suddenly catches fire without the supply of heat or friction externally is known as spontaneous combustion. 

The correct statement would be Spontaneous combustion is a type of combustion in which the substance suddenly catches fire without the supply of heat or friction externally.