A type of magnetic disk, or possibly magneto-optical disk which is not permanently attached to the disk drive (not a fixed disk) but which can be taken out and replaced, allowing many disks to be used in the same drive.
The term "removable disk" would seem to be applicable to floppy disks but is generally reserved for hard disks in suitable cartridges such as those made by Syquest, Iomega and others.
Removable disk packs were common on minicomputers such as the PDP-11 in use in the 1970s except that the drives were the size of washing machines and the disk packs as big as car wheels. Removable disks became popular on microcomputers in the 1990s as a cheap way of expanding disk space, transporting large amounts of data between computers and storing backups. Large, cheap fixed hard disks and USB memory sticks have made removable disks less attractive.