In the context of an
explosion, an area of greater-than-
standard air pressure, caused by
shock wave. Typically, a shock wave will produce areas of both
static and
dynamic overpressure.
Static overpressure is a condition wherein the ambient pressure (force exerted from all sides by the surrounding medium) is higher than standard.
Dynamic overpressure is the lateral force exerted by the mass of high-pressure
medium in the wave as it moves past - in the
atmosphere, this would
manifest as a wind.
In the case of humans, overpressure is not what usually directly kills those caught near a blast. The human lungs can withstand up to around 30 p.s.i. before being permanently damaged - that's approximately 16 p.s.i. of static overpressure. 20 p.s.i. of dynamic overpressure equates to a wind with a force of around 260 mph. This usually won't itself kill; however, trauma from being flung into objects or surfaces, or impacts from airborne debris, will.
Overpressure is most destructive to structures. Most of the famous film clips of structures being destroyed by nuclear detonations depict the common sequence of events: First, the static overpressure experienced as the shock wave envelopes the structure causes it to buckle inwards. Then, the dynamic overpressure following behind the wave front 'pushes' the structure, usually causing it to come apart and/or topple; finally, the negative pressure of the air rushing back in to fill the low-pressure zone behind the shockwave will whipsaw the structure back the other way. At that point, it will usually be unrecognizable as a house, or building, what-have-you.