Turing degree
Two sets of natural numbers are Turing equivalent if each is Turing reducible to the other. The notation "A" ≡T "B" indicates "A" and "B" are Turing equivalent. The relation ≡T is an equivalence relation known as Turing equivalence. A Turing degree is a collection of sets of natural numbers, such that any two sets in the collection are Turing equivalent. Equivalently, a Turing degree is an equivalence class of the relation ≡T.