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