ðɪs kˈɑːnsɛpt ɪz klˈoʊsli ɹᵻlˈeɪɾᵻd tə ðə kˈɑːnsɛpt ʌv ˈɪndɛks fɔːɹ dˌɪfɚɹˈɛnʃəl ˌældʒɪbɹˈeɪɪk ᵻkwˈeɪʒənz dˈiːz, wˌɪtʃ ɪz ðə nˈʌmbɚɹ ʌv tˈaɪmz ðɪ ˌældʒɪbɹˈeɪɪk ᵻkwˈeɪʒənz əvə dˌiːˌeɪˈiː mˈʌst biː dˌɪfɚɹˈɛnʃɪˌeɪɾᵻd ɪn ˈɔːɹdɚ tʊ əbtˈeɪn ɐ kəmplˈiːt sˈɪstəm ʌv dˌɪfɚɹˈɛnʃəl ᵻkwˈeɪʒənz fɔːɹ ˈɔːl vˈɛɹɪəbəlz.
This concept is closely related to the concept of index for differential algebraic equations (DAE's), which is the number of times the algebraic equations of a DAE must be differentiated in order to obtain a complete system of differential equations for all variables.