The definition of excession
given in the novel sparks a really neat idea. Basically, and i don't think i'm spoiling the novel at all here, an excession is what a society
simply cannot cope with. Something so completely beyond the scope of the experience and understanding of a culture
that encountering it is likely to result in the destruction
or drastic modification
of the culture. Another term used is "Out Of Context Problem
The example given, roughly, is of a comfortable existence developed on your own little island, where you've just about figured out what you can eat and what you can't, you've settled down to not entirely exhausting your food supply, you've mastered fire for cooking, spears for hunting, perhaps a little agriculture and some fishing, and everything looks hunky-dory for the continuation of a stable society. And then these large ships show up on the horizon filled with men with firesticks and a fiendishly effective way of cutting down trees and their priests really want to have a nice long chat with your tribal shaman. You have encountered an excession.
Now, think about what would constitute an excession for a spacefaring society like The Culture in Iain Banks' novels...