Conclusive, decisive, determinative, definitive are comparable when they mean having or manifesting qualities that bring something to a finish or end.
Conclusive applies most frequently to an argument, evidence, or reasoning that is irrefutable or so convincing that it compels certainty or certitude and puts an end to question or debate concerning a matter.
Decisive (see also DECIDED) applies to something (as an act, event, influence, or argument) that puts an end to controversy or competition, to vacillation, to uncertainty, or to insecurity; it often comes close in meaning to critical.
Determinative applies especially to matters (as decisions, judgments, operative causes, or influences) which put an end to uncertainty, wavering, and fluctuation and serve to give a fixed direction, goal, or character (as to a life, a course, or a movement).
Definitive, which is often opposed to tentative and, sometimes, to provisional, applies to whatever is put forth as final and as serving to make further questioning, dispute, uncertainty, or experiment needless or as serving to put an end to an unsettled state or condition where temporary measures have been necessary.