Decided, decisive are often confused, especially when they mean positive and leaving no room for doubt, uncertainty, or further discussion.
In this sense the words are applied chiefly to persons, their natures, their utterances or manner of utterance, their opinions, or their choices.
Decided implies a contrast with what is undetermined, indefinite, and neither this nor that; thus, a decided blue raises no question of its greenness or blackness; a decided success so far overpasses the line between success and failure that no one can question its favorable termination; a decided answer leaves no doubt of a person’s meaning, wishes, or intentions.
When applied to a person’s character, expression, or movements decided suggests such qualities or outward signs of qualities as determination, resolution, and lack of all hesitation or vacillation.
Decisive, on the other hand, implies an opposition to what is unsettled, uncertain, or wavering between this and that (for this sense as applied to things see CONCLUSIVE).
When used in reference to persons it implies ability or intent to settle or success in settling a controverted matter once and for all.