Kairos is an ancient Greek word that means "the right, critical, or opportune moment". It is one of two words that the ancient Greeks had for "time"; the other being chronos. In modern Greek, kairos also means "weather" or "time". Kairos is a tool that the ancient Greeks used to explain and understand the interposition of humans for their actions and the due consequences. In rhetoric, kairos is "a passing instant when an opening appears which must be driven through with force if success is to be achieved". It means that one must find the best situation, taking timing into consideration, to act. Kairos is also an alternate spelling of the name of the minor Greek deity Caerus, the god of luck and opportunity.

Kairos is about timeliness, appropriateness, decorum, symmetry, balance, and being aware of the rhetorical situation. It is about crafting serendipity, like when the sun comes out at the end of a romantic comedy after all the conflicts are resolved. Kairos represents a kind of "qualitative" time, as in "the right time". It means taking advantage of or even creating a perfect moment to deliver a particular message. Chronos, on the other hand, represents a kind of "quantitative" time, as in "What time is it?" or "Will we have enough time?".

Kairos is a principle that can be used in writing and communication to persuade an audience. To skillfully employ kairos, one must examine the rhetorical situation, consider the order and timing of the text, and be accommodative by appealing to each specific context. Being knowledgeable of and involved in the environment where the situation is taking place is also important to benefit fully from seizing the opportune moment.