Church-Turing thesis
Hypothesis: everything intuitively "effectively computable" can be computed by a Turing machine (or lambda calculus, which is equivalent). Not a provable theorem — a definition of "computable".
Church + Turing independently (1936). Consequence: all "reasonable" models of computation (Turing machines, lambda calculus, register machines, recursive functions, cellular automata, Conway's Life) are equivalent. Modern extension: Strong Church-Turing thesis — all effectively computable functions are polynomial-time equivalent. Quantum computing challenges the Strong version (Shor's algorithm factors in polynomial time; classical methods can't).