fɚɹə dˌiːvˌiːˈaɪ, ðɪ ˈɪndɛks ɪz ðə nˈʌmbɚɹ ʌv dˌɪfɚɹˌɛnʃɪˈeɪʃənz ʌv ˈɛf tˈiː, ˈɛks, jˈuː ˈiːkwəlz zˈiəɹoʊ nˈiːdᵻd ɪn ˈɔːɹdɚ tə lˈoʊkəli juːnˈiːkli aɪdˈɛntᵻfˌaɪ jˈuː æz ɐ fˈʌŋkʃən ʌv tˈiː ænd ˈɛks.
For a DVI, the index is the number of differentiations of F(t, x, u) = 0 needed in order to locally uniquely identify u as a function of t and x.