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

command line option
option

    (Or "option", "flag", "switch", "option switch") An
   argument to a command that modifies its function rather than
   providing data.  Options generally start with "-" in Unix or
   "/" in MS-DOS.  This is usually followed by a single letter
   or occasionally a digit.  More recently, GNU software
   adopted the --longoptionname style, usually in addition to
   traditional, single-character, -x style equivalents.

   Some commands require each option to be a separate argument,
   introduced by a new "-" or "/", others allow multiple option
   letters to be concatenated into a single argument with a
   single "-" or "/", e.g. "ls -al".  A few Unix commands
   (e.g. ar, tar) allow the "-" to be omitted.  Some options
   may or must be followed by a value, e.g. "cc prog.c -o prog",
   sometimes with and sometimes without an intervening space.

   getopt and getopts are commands for parsing command line
   options.  There is also a C library routine called getopt
   for the same purpose.

   (2007-02-18)