Kairos () is an ancient Greek word meaning the right or opportune moment (the 'supreme moment'). The ancient Greeks had two words for time: Greek: χρόνος (chronos) and kairos. While the former refers to chronological or sequential time, the latter signifies a period or season, a moment of indeterminate time in which an event of significance happens. What is happening when referring to kairos depends on who is using the word. While chronos is quantitative, kairos has a qualitative, permanent nature.Kairos also means weather in modern Greek. The plural, καιροί (kairoi (ancient and modern Greek)) means the times.
In Onians' 1951 etymological studies of the word, he traces the primary root back to the ancient Greek association with both archery and weaving. In archery, kairos denotes the moment in which an arrow may be fired with sufficient force to penetrate a target. In weaving, kairos denotes the moment in which the shuttle could be passed through threads on the loom.
In rhetoric, kairos is "a passing instant when an opening appears which must be driven through with force if success is to be achieved."
Kairos was central to the Sophists, who stressed the rhetor's ability to adapt to and take advantage of changing, contingent circumstances. In Panathenaicus, Isocrates writes that educated people are those “who manage well the circumstances which they encounter day by day, and who possess a judgment which is accurate in meeting occasions as they arise and rarely misses the expedient course of action".
Kairos is also very important in Aristotle's scheme of rhetoric. Kairos is, for Aristotle, the time and space context in which the proof will be delivered. Kairos stands alongside other contextual elements of rhetoric: The Audience, which is the psychological and emotional makeup of those who will receive the proof; and To Prepon, which is the style with which the orator clothes the proof.