The Free On-line Dictionary of Computing (30 December 2018):

floppy disk
crunchy
diskette
double density
floppy
HD
high density

    (Or "floppy", "diskette") A small,
   portable plastic disk coated in a magnetisable substance used
   for storing computer data, readable by a computer with a
   floppy disk drive.  The physical size of disks has shrunk from
   the early 8 inch, to 5 1/4 inch ("minifloppy") to 3 1/2 inch
   ("microfloppy") while the data capacity has risen.

   These disks are known as "floppy" disks (or diskettes) because
   the disk is flexible and the read/write head is in physical
   contact with the surface of the disk in contrast to "hard
   disks" (or winchesters) which are rigid and rely on a small
   fixed gap between the disk surface and the heads.  Floppies
   may be either single-sided or double-sided.

   3.5 inch floppies are less floppy than the larger disks
   because they come in a stiff plastic "envelope" or case, hence
   the alternative names "stiffy" or "crunchy" sometimes used to
   distinguish them from the floppier kind.

   The following formats are used on IBM PCs and elsewhere:

    Capacity  Density  Width
     360K	   double   5.25"
     720K	   double   3.5"
     1.2M	   high	    5.25"
    1.44M	   high	    3.5"

   Double denisty and high density are usually abbreviated DD and
   HD.  HD 3.5 inch disks have a second hole in the envelope and
   an overlapping "HD" logo.

   (1996-08-23)