How to pronounce undecidable:
Incapable of being algorithmically decided in finite time. for example, a set of strings is undecidable if it is impossible to program a computer (even one with infinite memory) to determine whether or not specified strings are included.
(of a wff) logically independent from the axioms of a given theory; i.e., that it can never be either proved or disproved (i.e., have its negation proved) on the basis of the axioms of the given theory. (note: this latter definition is independent of any time bounds or computability issues, i.e., more platonic.)