kairos

thumb|Kairos relief, copy of Lysippos, in [[Trogir (Croatia)]] thumb|Kairos as portrayed in a 16th-century fresco by Francesco de' Rossi (Il Salviati)|Francesco Salviati

Kairos () is an ancient Greek word meaning 'the exact or critical time'. In modern Greek, kairos also means 'weather' or 'time'. It is one of two words that the ancient Greeks had for 'time'; the other being (). Whereas the latter refers to chronological or sequential time, kairos signifies a good or proper time for action. In this sense, while is quantitative, kairos has a qualitative, permanent nature.