PSA
"Floppy disk" (with a "k")
A single thin and flexible plastic disk coated with a magnetic medium, inside a plastic shell. Yes, all variants are floppy disks, including 3.5", 5.25", 8" and some rarer variants. Removable.
"Hard disk" (with a "k")
One or more hard platters [usually made from metal or glass], coated with a magnetic medium, inside a rigid, air-tight shell. Usually not removable.
"Optical disc" (with a "c")
A single rigid plastic multi-layered disc with one or more metal layers that contain data. Most don't have a shell, though some variants do. Removable.
The disk/disc distinction is by convention.