An oracle AI is an AI system that only answers questions. It is a proposed solution to the AI alignment problem, apparently first suggested by Nick Bostrom.
Further reading
Bostrom, Nick (2014) Superintelligence: Paths, Dangers, Strategies, Oxford: Oxford University Press, pp. 145–148.
Muehlhauser, Luke (2012) A taxonomy of oracle AIs, LessWrong, March 8.