The designation for the primary hard disk in a Windows PC. The C: evolved from the early personal computers that used A: and B: for their two floppy drives. When the hard disk came along, it naturally became the C: drive. Letters starting with D: and up are used for additional hard disks and CD/DVD drives.
Even though floppy drives no longer come with computers, A: and B: are still reserved, and there is the occasional need for a floppy drive to read an old disk. See logical drive