May be defined as an inert material of some strength and having a given wall thickness, situated in the immediate vicinity of an explosive.

Priming or heating the explosive materials produces different results, according to whether they are located in stronger or weaker confinement. If confined by thick steel, almost any explosive will explode or detonate on being heated; on the other hand, they burn on contact with an open flame if unconfined combustion; Mass explosion risk), except initiating Explosives.

The destructive (fragmentation) effect of an explosion becomes stronger if the explosive is confined (stemmed) in an enclosure such as a borehole. In the absence of natural confinement, the explosive charge is often embedded in an inert material such as clay.

The process by which some explosives, e.g., a blackpowder, can change from extremely rapid burning to something approaching detonation. For instance, blackpowder confined in a tube will produce a loud report when lit, whilst blackpowder burning loose does not.