A theory is decidable iff there is an algorithm which can determine whether or not any sentence r is a member of the theory.

Church-Turing thesis | deterministic | Gödel's completeness theorem | Gödel's first incompleteness theorem | Gödel's second incompleteness theorem | Kreisel conjecture | recursively undecidable | Rice's theorem | Richardson's theorem | sentence | Tarski's theorem | theory | undecidable