A transcoder is a product or system which transforms text from one format to another for the purposes of presentation.  It's some times called Content Transformation.  The word 'transcoder' has also been mentioned in terms of converting character encoding (i.e. ISO to Cyrillic, etc).  Generally speaking, a transcoder sits between an application server or web server and the client (usually a browser).

A number of transcoders exist, but there are multiple definitions of the word 'transcoder'.  Note that a transcoder is distinct from application level middleware such as CaiTdm, which operates on the back end of a system rather than the front even though it performs a similar operation.

The primary use of transcoders in my experience is to convert from HTML to WML.
This is typically done either directly (using a recursive-descent parser) or by converting into an XML format and doing an XSL or XSLT transformation on the document.

Because WML is currently such a dog's breakfast when it comes to implementation, most people never actually write WML pages directly -- they write XML and do an XSLT conversion for whatever device they need to target.

Examples of transcoders:
* SpyglassPrism
* DigitalPlastic
* IBMTranscodingPublisher
* SiteMorfer (dunno about this one)

----
For me, the term is clearly restricted to converting between ''encodings'' (Latin1 -> UTF-8), not ''representations'' (XML -> WML). What you describe above is better keyed as a transformation. Transcoding really just changes ''encoding'', not ''content''; i.e. it changes syntax, not semantics. -- JuergenHermann

----
For me the term is just the superposition: a transcoder is an encoder and a decoder in one component. 
-- GunnarZarncke

So, a long word for codec?  :p