Kairos: a time when conditions are right for the accomplishment of a crucial action; the opportune and decisive moment.
From Ancient Greek: καιρός (kairós).
Kairos is the fourth rhetorical appeal, but it is almost always best explained in context with the other rhetorical appeals.
In his article “Critical-Rhetorical Ethnography: Rethinking the Place and Process of Rhetoric,” Aaron Hess provides a definition of kairos for the present day that bridges the two classical applications. Hess addresses Poulakos’s view that, “In short, kairos dictates that what is said must be said at the right time.” He suggests that kairos also considers appropriateness. According to Hess, kairos can either be understood as “the decorum or propriety of any given moment and speech act, implying a reliance on the given or known” or as “the opportune, spontaneous, or timely.”
Kairos is an important part of appeals to logos because all logical information is contextual. In other words, you can not use data from a 1960s study about cancer treatment without contextualizing it: it is no longer timely in 2017.
The appropriateness of information is also related to kairos. Even if information is logical and factually correct, it may not always be timely to present it. You must evaluate the rhetorical situation in order to balance the appeals appropriately, and an important part of the rhetorical situation is the time.