An ASCII control or metacharacter (#27, ^]) with its own key on most keyboards, intended originally to signify escape (v) (sense 1). While it has been put to a number of different uses over the decades, it is still often used to pause or terminate a program or process. Frequently called Meta in some programs, notably emacs, where it is a common command prefix.
To pause a running program and return control temporarily to the operating system, usually in order to run some other program. In Unix, the exclamation point (ASCII #33, !, pronounced "bang") is an escape character that can be used in most programs to accomplish this. To cancel the default ( meta-)interpretation of the following character in a string and interpret it literally instead. Thus, while the unescaped (meta)expression " ." matches any character, the regular expression "\." matches a literal period or full stop character only, because it is escaped by the preceding "\".
From Extensible Markup Language (XML) 1.1 ( 2004-02-04) Entity and character references MAY both be used to escape the left angle bracket, ampersand, and other delimiters. A set of general entities (amp, lt, gt, apos, quot) is specified for this purpose. Numeric character references MAY also be used; they are expanded immediately when recognized and MUST be treated as character data, so the numeric character references and &MAY be used to escape and & when they occur in character data.