Wikipedia
Excession, first published in 1996, is Scottish writer Iain M. Banks's fifth science fiction novel to feature the Culture, a fictional interstellar society. It concerns the response of the Culture and other interstellar societies to an unprecedented alien artifact, the Excession of the title.
The book is largely about the response of the Culture's Minds ( AIs with enormous intellectual and physical capabilities and distinctive personalities) to the Excession itself and the way in which another society, the Affront, whose systematic brutality horrifies the Culture, tries to use the Excession to increase its power. As in Banks' other Culture novels the main themes are the moral dilemmas that confront a hyperpower and how biological characters find ways to give their lives meaning in a post-scarcity society that is presided over by benign super-intelligent machines. The book features a large collection of Culture ship names, some of which give subtle clues about the roles these ships' Minds play in the story. In terms of style, the book is also notable for the way in which many important conversations between Minds resemble email messages complete with headers.
Usage examples of "excession".
Around the Excession itself, no sign whatsoever of the vanished Stargazer fleet of the Zetetic Elench.
Excession, the Affront are just the sort of species - and at precisely the most likely stage in their development - to attempt some sort of mad undertaking which, however likely to fail, if it did succeed might offer rewards justifying the risk.
The Excession is still the more important matter, but its appearance and the possibilities it may open up have been used by the conspiracy to tempt the Affront into initiating hostilities.
Psychopath ROUs are bound for the Excession but the rest are down for defensive duties elsewhere to cope with likely threats from Affront battle units.
Then they dump the boring, pedantic GSV that happened to be on the Incident Coordinating Rota, agree to wait-and-see with the Excession itself while sending investigatory reinforcements, start a localised mobilisation - mobilisation!
The point is that the war itself takes precedence, with the Excession prioritised beyond that.