The Micro-Disk (also labelled as Mikro-Disk or Micro-Disc) was a small floppy disk design used by Olympia for some of its electronic typewriters such as some models in the Supertype, Startype and Disque ranges.
Essentially it is a version of the single-sided 2.5-inch mylar Olivetti minidisc but instead of just a bare disk, the Olympia version houses it in a holder that helps protect the magnetic surface from fingerprints and also allows for a label to be inserted. It appears that the Olympia version held 8 KB of data.
It seems to have been introduced in 1977, the same year as the Olivetti minidisc, and the Olympia Micro-Disk was used in a standalone drive that had one slot for the disk being read and four more that appear to have been just to store disks in. Later, it appears that some Olivetti typewriter models such as the Supertype 240 and Disque had built-in drives.