EMZETT.Decompressing
In short: Converting compressed data back into its original (or a usable) form — the counterpart to compressing.
In more detail: With lossless compression, this produces exactly the original again; with lossy compression (e.g. JPEG), the decompressed file is no longer identical to the very first source material, since information was already discarded during compression.
In Depth
With lossless formats (e.g. ZIP, 7-Zip, PNG), decompressing is purely mechanical: the algorithm exactly reverses the rules applied during compression (e.g. repeated byte sequences that were replaced by shorter codes are expanded again) — the original is reproduced byte for byte, verifiable via a checksum.
With lossy formats (JPEG, MP3, MPEG), “decompressing” is actually imprecise: the information discarded during compression (e.g. colour nuances barely perceptible to the human eye in JPEG) can’t be recovered. What happens when opening such a file is more precisely a reconstruction of as faithful an approximation as possible from the remaining data — which is why quality keeps degrading with repeated compressing/decompressing of the same file (e.g. saving a JPEG several times), with every pass.
The computational cost is asymmetrically distributed for most common algorithms: decompressing is usually considerably faster than compressing, because compressing has to search for optimal patterns, while decompressing just works through a fixed mapping table.
Zip bombs as an attack vector
A well-known abuse case of decompressing is so-called “zip bombs” — tiny archive files, only a few kilobytes, that grow to several gigabytes or even terabytes when unpacked, by exploiting extreme compression ratios and nested archives-within-archives. Automated systems that blindly unpack incoming files (e.g. virus scanners or file-upload processing) can be specifically brought to their knees this way, which is why secure implementations should enforce size limits and timeouts when unpacking, instead of decompressing files without limit.
See also: Compressing, ZIP, 7-Zip