We say that a function is A-computable if it can be computed by a Turing machine equipped with oracle A. We define that B is Turing computable relative to A, written B ≤TA, if the characteristic function of B, having value 1 for members of B and 0 otherwise, is computable from a machine with oracle A. Two oracles, A and B, are Turing equivalent if each is computable relative to the other. This is an equivalence relation, whose equivalence classes are called Turing degrees.
