A hypothetical super-intelligent being that perfectly implements Bayesian rationality. Omega is most commonly used as the predictor in Newcomb's problem. In its role as predictor, Omega's predictions occur almost certainly.
Omega can be seen as analogous to Laplace's demon, or as the closest approximation to the Demon capable of existing in our universe.
In some thought experiments, Omega is also taken to be super-powerful. (Arguably this or something like it, in addition to superintelligence, is required for it to play its prescribed role in Newcomb's problem.) In these cases, Omega is roughly equivalent to the God of traditional theism except that no particular moral qualities are ascribed to it.