Outside Context Problem

As he states in the book:

"An Outside Context Problem was the kind of thing most civilisations encountered just once, and which they tended to encounter in the same way a sentence encountered a full stop."

More generally, an OCP could be described as a problem of great significance which one never actually thinks of, since one doesn't even the knowledge to realize such problem can arise.

For a small civilization dealing with their own problems, suddenly being threatened by much larger, uncomparably more sophisticated civilization would be an OCP. For imaginary two-dimensional beings, meeting a three-dimensional one could mean an OCP. There have been some examples of cultures encountering such problems in the course of human history, for example tribes of primitive islanders being visited by ships from advanced continental civilizations they were not previously aware of.

Warning: Wikipedia contains spoilers

In Excession, The Culture meets their own OCP embodied by a large black sphere sitting somewhere in space - the Excession. It appears to be linked to the very fabric of the known universe and is believed to be a connection with a whole lot of parallel universes. And also, it aggressively defends its right to remain unknown... or so it would seem.