Compression is the act of compressing. Decompression is the opposite act of compression.
I have taken the rather arcane nomenclature of a mathematic process, a form of computation, to denote the process of compression in a random or nonrandom sequential string as being a ‘compression engine’. This rather Victorian and mechanistic allusion to Babbage’s folly is, in essence, a simple computing operation that sorts and stores the original segment, in like groups, and then reduces each subgroup into a notational symbol of the original number of characters found in that specifi c subgroup (Note #1). Decompression, the act of restoring the original number and type of characters found in the original string.