Validator complains about <nobr>

My Doctype is XHTML 1.0 Transitional	

Why validator complains about <nobr>

Error  Line 156 column 11: element "nobr" undefined.

      <nobr>Folder</nobr>

You have used the element named above in your document, but the
document type you are using does not define an element of that name.
This error is often caused by:

    * incorrect use of the "Strict" document type with a document that
uses frames (e.g. you must use the "Frameset" document type to get the
"<frameset>" element),
    * by using vendor proprietary extensions such as "<spacer>" or
"<marquee>" (this is usually fixed by using CSS to achieve the desired
effect instead).
    * by using upper-case tags in XHTML (in XHTML attributes and
elements must be all lower-case.



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