These words are concerned with those moments in a sequence of events that are crucial to bringing about a given result. Conclusive suggests a final act that settles questions at issue in a previous course of action: the atomic-bombing of Hiroshima as the conclusive event of World War II. The word may be used in another sense to refer, not to the events themselves, but to a later analysis of them. In this sense, conclusive refers to an incontrovertible interpretation of these events as a whole: conclusive proof that Richard II was not the villain Shakespeare made him out to be. A decisive event differs from a conclusive event in that it need not necessarily be the last one in a given sequence. Indeed, it may be a situation present almost at the outset, with the whole following course of action merely corroborating it.

• The enemy’s lack of inspired leadership was decisive to its defeat, long before a single shot was fired.

Determining , like decisive , may refer to something that comes early or late in a course of action. Decisive may be more suitable in describe underlying factors that are inherent in a situation and prejudice the outcome.

• No matter how many decisive victories Justinian won, the split of East and West into two alien cultures was the determining factor in his ultimate failure to re-unite the Roman Empire.

Definitive , when referring to an act, combines the meanings of both conclusive and decisive . It applies when a sequence of events has been in doubt until the very last act, which irrevocably settles things: Elizabeth’s definitive victory over the Armada settled the question of British naval superiority for centuries. When the word refers to a later analysis of event, it takes in all that is implied by conclusive in this sense, but adds to it implications of authoritative accuracy and incontestable interpretation not to be surpassed. In this sense, definitive , unlike conclusive , need not suggest the settling of controversy or the proposing of any thesis whatsoever; it simply suggests finality of examination and analysis: the definitive history of the Boer War; the definitive biography of Byron that strips away all myth and conjecture to show us the actual man as he was.