It is a term from computer science or computability theory.You know,there are some sets that are not computable or recursive or not computably enumerable.To compare these sets,the Turing machine may ask information source outside the turing machine(you may think of turing machine as computer) the question about the membership for yes or no when it can not decide a element's membership or it can not decide a element's membership after amount of computation steps.To extend such an idea of information source to compare two computable sets ,one may be regarded as information source so as to decide the membership of other set's element.So it is defined in two cases,one is incomputable,the other is computable.
Because the information source outside the Turing Machine is not gotten by computation or reasoning,it comes to Turing machine as from God,we call it oracle,or oracle information.You know in ancient china,people used oracle to get God's intention