The reason for this is actually pretty interesting though. Historically it was just a US/UK English difference, but it evolved into both being used because one of the first big manufacturers of optical discs, Philips, called them discs, while the US-based IBM spelled their magnetic disks with a K.
So the Panopticon. The hypothetical prison that even people in the 1800s thought would be a human rights violation to build because it was such an extreme form of psychological torture.