ɐn ˈɔːɹdɪnəl ɪz ˈæstɚɹˌɪsk dᵻfˈaɪnəbəl fɹʌm ɐ klˈæs ʌv ˈɔːɹdɪnəlz ˈɛks ɪf ænd ˈoʊnli ɪf ðɛɹ ɪz ɐ fˈɔːɹmjʊlə ænd sˈʌtʃ ðæt ɪz ðə juːnˈiːk ˈɔːɹdɪnəl fɔːɹ wˌɪtʃ wˌɛɹ fɔːɹ ˈɔːl wiː dᵻfˈaɪn təbi ðə nˈeɪm fɔːɹ wɪðˈɪn .
An ordinal  is *definable from a class of ordinals X if and only if there is a formula  and  such that  is the unique ordinal for which  where for all  we define  to be the name for  within .