ɪɾ ɪz ˈɔːlsoʊ ɐ nˈoʊʃən klˈoʊs tə ðə ɹˈɛlətˌɪv dᵻɡɹˈiː ʌv kəntɹˈoʊl θˈiəɹi, wˈɪtʃ ɪz, ɹˈʌfli spˈiːkɪŋ, ðə nˈʌmbɚɹ ʌv tˈaɪmz æn "ˈaʊtpʊt" vˈɛɹɪəbəl hɐz təbi dˌɪfɚɹˈɛnʃɪˌeɪɾᵻd sˌoʊ ðˈæt æn "ˈɪnpʊt" vˈɛɹɪəbəl ɐpˈɪɹz ɛksplˈɪsɪtli ɪn kəntɹˈoʊl θˈiəɹi ðɪs ɪz jˈuːzd tə dᵻɹˈaɪv ɐ kɐnˈɑːnɪkəl stˈeɪt spˈeɪs fˈɔːɹm wˌɪtʃ ɪnvˈɑːlvz ðə sˌoʊkˈɔːld "zˈiəɹoʊdaɪnˈæmɪks", ɐ fˌʌndəmˈɛntəl kˈɑːnsɛpt fɔːɹ kəntɹˈoʊl.
It is also a notion close to the relative degree of Control Theory, which is, roughly speaking, the number of times an "output" variable has to be differentiated so that an "input" variable appears explicitly in Control Theory this is used to derive a canonical state space form which involves the so-called "zero-dynamics", a fundamental concept for control).