Meta is a prefix that in most information technology usages means "an underlying definition or description." Thus, metadata is a definition or description of data and metalanguage is a definition or description of language. Meta (pronounced MEH-tah in the U.S. and MEE-tah in the U.K.) derives from Greek, meaning "among, with, after, change." Whereas in some English words the prefix indicates "change" (for example, metamorphosis), in others, including those related to data and information, the prefix carries the meaning of "more comprehensive or fundamental."
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The Standard Generalized Markup Language (SGML) defines rules for how a document can be described in terms of its logical structure (headings, paragraphs or idea units, and so forth). SGML is often referred to as a metalanguage because it provides a "language for how to describe a language." A specific use of SGML is called a document type definition (DTD). A document type definition spells out exactly what the allowable language is. A DTD is thus a metalanguage for a certain type of document. (In fact, the Hypertext Markup Language (HTML) is an example of a document type definition. HTML defines the set of HTML tags that any Web page can contain.)
The Extensible Markup Language (XML), which is comparable to SGML and modelled on it, describes how to describe a collection of data. It's sometimes referred to as metadata. A specific XML definition, such as Microsoft's Channel Definition Format (CDF), defines a set of tags for describing a Web channel. XML could be considered the metadata for the more restrictive metadata of CDF (and other future data definitions based on XML).
In the case of SGML and XML, "meta" connotes "underlying definition" or set of rules. In other usages, "meta" seems to connote "description" rather than "definition." For example, the HTML tag is used to enclose descriptive language about an HTML page.
One could describe any computer programming or user interface as a metalanguage for conversing with a computer. And an English grammar and dictionary together could be said to define (and describe) the metalanguage for spoken and written English.