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

Macintosh file system
data fork
desktop database
resource fork

    A file on the Macintosh consists of two parts,
   called forks.  The "data fork" contains the data which would
   normally be stored in the file on other operating systems.
   The "resource fork" contains a collection of arbitrary
   attribute/value pairs, including program segments, icon
   bitmaps, and parametric values.  Yet more information
   regarding Macintosh files is stored by the Finder in a
   hidden file, called the "Desktop Database".

   Because of the complications in storing different parts of a
   Macintosh file in non-Macintosh file systems that only handle
   consecutive data in one part, it is common to only send the
   Data fork or to convert the Macintosh file into some other
   format before transferring it.

   (1996-03-03)