undecidable Syllabification: (un·de·cid·a·ble)Pronunciation: /ˌəndiˈsīdəbəl/Definition of undecidable adjective not able to be firmly established or refuted. Logic (of a proposition or theorem) not able to be proved or disproved. Derivatives undecidabilityPronunciation: /-ˌsīdəˈbilətē/ noun undecidable in other Oxford dictionaries Definition of undecidable in the British & World English dictionary