de;ci;sive pronunciation: function: adjective date: 1611 1 : having the power or quality of deciding lt;a decisive battlegt;2 : resolute, determined lt;a decisive mannergt;3 : unmistakable, unquestionable lt;a decisive superioritygt;synonyms see conclusive— de;ci;sive;ly adverb — de;ci;sive;ness noun