Geological concept that substantial change to the Earth's crust occurs both rapidly and to large degree disastrously.
Think of catastrophic volcanoes, earthquakes, landslides, and floods, particularly with destruction occurring that is much greater than the historical norm (i.e., what we are used to seeing). To a degree it is quite true that geological change can and does occur in the course of events such as these, but catastrophism represents an extreme position where it is solely these events that give rise to geological as well as evolutionary change.
That is, that geological change occurs in abrupt shifts literally in landscapes rather than occurring via less intense, gradual changes. Biologically, with catastrophism species would be thought to be wiped out, at least locally, and then are replaced, in the geological record, simply by species movement from other areas that had not been similarly affected. See by contrast Uniformitarianism.