An acronym for Amiga Disk File.
An ADF is a disk image, or raw copy of an Amiga floppy disk. ADF files can made using a piece of Amiga software known as TransDisk, which creates the file from a disk currently in the Amiga's floppy drive. The primary reason for creating ADF files is for use with an Amiga emulator such as WinUAE or Fellow, since normal PC floppy drive controllers are unable to read Amiga-formatted disks.
ADF files of normal double density Amiga disks are exactly 880k (901,102 bytes) in size.