Disk compression
Transparent compression of a whole disk volume to squeeze out more space — the 1990s answer to small, expensive hard drives.
When hard drives were measured in megabytes and priced accordingly, people compressed them on the fly. Stac Electronics' Stacker (1990) was the pioneer; Microsoft bundled DoubleSpace into MS-DOS 6.0 (1993), then switched to DriveSpace after Stac won a $120 million patent suit.
The technique intercepted disk I/O and packed data — usually LZ-based — into a single large container file, a "compressed volume". The OS saw an ordinary drive, roughly twice the size. It died out once drives got cheap and the filesystems after FAT gained built-in compression, and its reputation for losing everything when the container file corrupted didn't help. Related to but distinct from defragmentation, which reorders the same data rather than shrinking it.