**EDIT**   
I would think that:   
 ******uncompressed data* is data that was never altered  
 ******decompressed data* is data that has been compressed, then returned to its original state by the process of *decompression*.

Decompress and uncompress seem to be used synonymously in the context of computing, although decompress seems to be used more frequently. A search on MSDN for **[uncompress*][1]** returns 731 hits, while **[decompress*][2]** returns 4,450 hits.

Seaching on SO gives similar results. There are  

 - 22 questions tagged with **[uncompress][3]**
 - none tagged with uncompression, 
 - 651 occurrences of uncompress and 
 - 34 occurrences of uncompression.  
Total tag count = 22  
Total word count = 685

In contrast, there are 

 - 39 questions tagged with **[decompress][4]**
 - 110 questions tagged with **[decompression][5]** 
 - 1,467 occurrences of decompress and 
 - 865 occurrences of decompression.  
Total tag count = 149  
Total word count = 2,332


  [1]: http://social.msdn.microsoft.com/Search/en-US?query=uncompress*&beta=0&ac=8  "Query MSDN English for uncompress with wildcard suffix"
  [2]: http://social.msdn.microsoft.com/Search/en-US?query=decompress*&beta=0&ac=3 "Query MSDN English for decompress with wildcard suffix"
  [3]: http://stackoverflow.com/questions/tagged/uncompress "uncompress tag count on SO"
  [4]: http://stackoverflow.com/questions/tagged/decompress "decompress tag count on SO"
  [5]: http://stackoverflow.com/questions/tagged/decompression "decompression tag on SO"