An excession is a pejorative Culture term, sometimes used by Culture-ulterior groups like the Zetetic Elench. It is used to describe things which are dangerous to an excessive point.
"Excession; that was what the Culture called such things. It had become a pejorative term and so the Elench didn't use it normally, except somtimes informally, amongst themselves. Excession; something excessive. Excessively aggressive, excessively powerful, excessively expansionist; whatever. Such things turned up or were created now and again. Encountering an example was one of the risks you ran when you went a-wandering."
- Dn. Sisela Ytheleus 1/2, Excession.