Sydex TeleDisk - "The Diskette Fax Machine" was an MS-DOS utility for encoding floppy disks that allowed non-MS-DOS standard disks to be read, archived or transferred and then recreated on another machine.
Teledisk is a program which reads a floppy diskette into an image file on the PC, and can recreate a nearly exact copy of the floppy at a later date from that image file. For many years, Teledisk was the only program like this which could handle non-PC formats, and it became the primary method by which people preserved and exchanged system disks for classic computers.
Originally sold as a shareware product by a company called Sydex, all rights to Teledisk were later sold to NTI, who no longer supports it or offers it for sale. This puts Teledisk into a state of limbo, as you cannot legally obtain or use later NTI versions, and the Sydex shareware versions cannot be used beyond the evaluation period without a purchased license, which you currently cannot buy.
The format died before the Windows 95 era due to bit rot but can now the TD0 files can be easily converted using SamDisk Utility.