One-point safe criterion
For nuclear weapons, the one-point safe criterion is a protection against accidental explosion. A weapon that meets the criterion will not produce an explosive yield of more than two kilograms if an explosive, either part of its own implosion system or an externally applied one, is detonated in contact with the "physics package".
It has been reported that all operational U.S. nuclear weapons pass the one-point safe criterion, and this is probably a goal for the other nuclear powers. The first test of the criterion, for a fully assembled bomb, was the Pascal-A test in 1957, which produced an unacceptable yield of 55 tons. It is unclear when success was first achieved, but it is known that the Starfish test in 1962, launched with a PGM-17 Thor missile that had to be destroyed when it went off course, was destroyed by the onboard demolitions charges without any nuclear yield.
Without an unusual safety mechanism, the British Violet Club was not one-point safe.
One of the means of achieving the criterion is the use of insensitive high explosives. Implosion systems are built of a large number of highly directional detonators. If one went off, there would not be enough energy to set off the implosion charge under an adjacent detonator. The implosion system requires precise time synchronization of the detonations (e.g., using krytron switches), so even a delayed one-point detonation would not allow the symmetrical compression wave to form.
An improvised terrorist bomb, however, achieves terror even if it goes off somewhere other than the target, so such weapons cannot be assumed to meet the criterion.