Usually, it signifies that what is to follow is from a different character code, i.e., have meanings other than normal.

• In alphabet coding schemes, a specially designated character, the occurrence of which in the data signifies that one or more of the characters to follow are from a different character code, i.e., have meanings other than normal.
• In a text-control sequence of characters, a control character that indicates the beginning of the sequence and the end of any preceding text. This is for instance used to control character terminals and printers.
• In the ASCII character code, the character ESC with decimal code 27 and hexadecimal code 1B.