A ghost civilization is a technological society of beings that are descended from intelligent life, but is not intelligent life anymore. For some reason they lost their sapience through evolution. This means that they cannot create new falsifiable hypotheses nor quite understand the appliceability and their bounds of old falsifiable hypotheses and theories. They can, however, follow instructions and keep using technology, and possibly invent some new tools though the general trend will be that technology is slowly lost. A ghost civilization is, thus, a transitional stage between an intelligent past and a savage future.
Losing sapience Edit
How intelligent life can lose its sapience is interesting, in a scary and horrible sort of way. The traditional Idiocracy scenario of idiots reproducing more in the absence of selection for intelligence is unlikely to be sufficient on its own, since assortative mating can retain a sapient "species within a species". If the sapients are actively persecuted, the result is much more likely to be a ghost covilization.
This persecution can take place by rather Earthlike means, such as laws that specifically punish intentional actions and/or moralities that specifically condemn intentional actions. Selectively punishing individuals with characteristic X selects against characteristic X, it is an universal law of evolution. Not punishing individuals that "cannot help" what they are doing selects for non-sapience by means of positive selection of characteristics that entail impunity in society.