In modern usage, an ESC sent to the terminal usually indicates the start of a command sequence usually in the form of a so-called "ANSI escape code" (or, more properly, a "Control Sequence Introducer") from ECMA-48 (1972) and its successors, beginning with ESC followed by a "[" (left-bracket) character.
ɪn mˈɑːdɚn jˈuːsɪdʒ, ɐn ˌiːˌɛssˈiː sˈɛnt tə ðə tˈɜːmɪnəl jˈuːʒuːəli ˈɪndᵻkˌeɪts ðə stˈɑːɹt əvə kəmˈænd sˈiːkwəns jˈuːʒuːəli ɪnðə fˈɔːɹm əvə sˌoʊkˈɔːld "ˈænsi ɛskˈeɪp kˈoʊd" ɔːɹ, mˈoːɹ pɹˈɑːpɚli, ˈeɪ "kəntɹˈoʊl sˈiːkwəns ˌɪntɹədˈuːsɚ"fɹʌm ˈɛkmə fˈoːɹɾi ˈeɪt nˈaɪntiːnhˈʌndɹɪd sˈɛvənti tˈuː ænd ɪts səksˈɛsɚz, bɪɡˈɪnɪŋ wɪð ˌiːˌɛssˈiː fˈɑːloʊd baɪ ˈeɪ "" lˈɛftbɹˈækɪt kˈæɹɪktɚ.