Compact Floppy Disk
The three-inch Compact Floppy Disk. Amdek was a big vendor of Compact Floppy drives for the popular U.S. 8-bits (Apple II and Atari, anyway).
I grabbed such a drive and a box of blank Compact Floppies for cheap back in the late 80s for an Apple IIe of mine. It plugged into the standard Disk ][ controller and offered 140K per side (they are "flippy").
These are at least twice as thick as today's (well...yesterday's?) 3.5-inch microfloppies.