\newcommand{cmd}[args]{def}
  \renewcommand{cmd}[args]{def}

  These commands define (or redefine) a command.

   -  cmd:  A command name beginning with a \.  For \newcommand it must
      not be already defined and must not begin with \end; for
      \renewcommand it must already be defined.

   -  args:  An integer from 1 to 9 denoting the number of arguments of
      the command being defined.  The default is for the command to
      have no arguments.

   -  def:  The text to be substituted for every occurrence of cmd; a
      parameter of the form  n in cmd is replaced by the text of the
      nth argument when this substitution takes place.