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The term CDATA, meaning character data, is used for distinct, but related, purposes in the markup languages SGML and XML. The term indicates that a certain portion of the document is general character data, rather than non-character data or character data with a more specific, limited structure.

In an XML document or external parsed entity, a CDATA section is a section of element content that is marked for the parser to interpret purely as textual data, not as markup. A CDATA section is merely an alternative syntax for expressing character data; there is no semantic difference between character data that manifests as a CDATA section and character data that manifests as in the usual syntax in which, for example, "<" and "&" would be represented by "<" and "&", respectively.

A CDATA section starts with the following sequence:

and ends with the next occurrence of the sequence:

All characters enclosed between these two sequences are interpreted as characters, not markup or entity references. In:

the start and end "sender" tags are interpreted as markup. However, if written like this:

then the code is interpreted the same as if it had been written like this:

That is, the "sender" tags will have exactly the same status as the "John Smith" — they will be treated as text.

Similarly, if the numeric character reference ð appears in element content, it will be interpreted as the single Unicode character 00F0 (small letter eth). But if the same appears in a CDATA section, it will be parsed as six characters: ampersand, hash mark, digit 2, digit 4, digit 0, semicolon.

New authors of XML documents often misunderstand the purpose of a CDATA section, mistakenly believing that its purpose is to "protect" data from being treated as ordinary character data during processing. Some APIs for working with XML documents do offer options for independent access to CDATA sections, but such options exist above and beyond the normal requirements of XML processing systems, and still do not change the implicit meaning of the data. Character data is character data, regardless of whether it is expressed via a CDATA section or ordinary markup. CDATA sections are useful for writing XML code as text data within an XML document. For example, if one wishes to typeset a book with XSL explaining the use of an XML application, the XML markup to appear in the book itself will be written in the source file in a CDATA section.

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