Turing degree
Informally, a set of natural numbers "A" is "Turing reducible" to a set "B" if there is an oracle machine that correctly tells whether numbers are in "A" when run with "B" as the oracle set (in this case, the set "A" is also said to be ("relatively") "computable from" "B" and "recursive in" "B"). If a set "A" is Turing reducible to a set "B" and "B" is Turing reducible to "A" then the sets are said to have the same "Turing degree" (also called "degree of unsolvability"). The Turing degree of a set gives a precise measure of how uncomputable the set is.