Currently we have both:
compression × 185 (19 asked this year)
the process in which a file or folder's size is reduced by using an algorithm to encode the same information using fewer bits.
decompress × 21 (1 asked this year)
I think we can safely get rid of decompress by retagging all questions with compression instead. Probably we should add a tag synonym like decompression instead though, to prevent people from recreating that tag.
Opinions? I'd be happy to edit those 21 questions and change the tags if there's positive feedback, but I can't currently suggest synonyms on compression.