When on, the ":substitute" flag 'g' is default on. This means that all matches in a line are substituted instead of one. When a 'g' flag is given to a ":substitute" command, this will toggle the substitution of all or one match. See complex-change.

    command         'gdefault' on   'gdefault' off
    :s///             subst. all      subst. one
    :s///g            subst. one      subst. all
    :s///gg           subst. all      subst. one

NOTE: This option is reset when 'compatible' is set. Setting this option may break plugins that rely on the default behavior of the 'g' flag. This will also make the 'g' flag have the opposite effect of that documented in :s_g. This option is not used in Vim9 script.

(default off)