Difference between "uncompress" and "decompress"

Solution 1:

decompress — that's it.

Uncompressed = not compressed; may never have been compressed in the first place. Just plain data is uncompressed data in every sense. To specifically refer to that which has undergone the process of decompressing, we would say decompressed data. Looking at it in another way, data before being compressed is uncompressed — that extracted from compressed format is decompressed data.

[Edit-1] Also, per Alenanno,
decompressed is a past participle of the verb decompress. Uncompressed is an adjective.

Solution 2:

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* returns 731 hits, while decompress* returns 4,450 hits.

Seaching on SO gives similar results. There are

• 22 questions tagged with uncompress
• 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
• 110 questions tagged with decompression
• 1,467 occurrences of decompress and
• 865 occurrences of decompression.
  Total tag count = 149
  Total word count = 2,332