A metacharacter is a term often used in relation to various kinds of regular expressions to mean a short-cut name for something else.
In general, it refers to a character that is involved in the processing of the message itself and not of the content of the message, taking “message” in a generalised sense. For example, the syntax rules described in the Tcl.n dodecalogue describes Tcl's metacharacters; the remaining manual pages describe the content of the “messages”, which in Tcl are command invocations. (Some of these recursively use the basic Tcl syntax — e.g., if, while, proc — but which do and which don't is a feature of the particular commands.)
AMG: See [L1 ] for code to selectively inhibit the special interpretation of regular expression metacharacters.