Kairos

εἰ δ᾽ ὁ καιρὸς ἦν σαφής,

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"If propriety (kairos) were more clear," says Phaedra, "two things would not be carried by the same letters."  

Kairos is a rich and complex word, which in itself exemplifies the problem of multiple meanings being "carried by the same letters." The notion of fitness, applied to time, suggests something that is seasonable, opportune, and from there it can extend to the notion of a "kairotic moment," a critical turning point, a climactic moment either to be seized or lost forever.

καιρός , ὁ,
A. due measure, proportion, fitness (not in Hom.), καιρὸς δ᾽ ἐπὶ πᾶσιν ἄριστος (which became a prov.) Hes.Op.694,Thgn. 401; “κ. παντὸς ἔχει κορυφάν” Pi.P.9.78; “κ. Χάριτος” A.Ag.787 (anap.) (cf. “ὑποκάμπτω” 11); εἰ ὁ κ. ἦν σαφής the distinction, the point, E.Hipp. 386; ἡ ἀπορία ἔχει τινὰ κ. has some point or importance, Arist. Metaph.1043b25;καιροῦ πέρα beyond measure, unduly, A.Pr.507; “μείζων τοῦ κ. γαστήρ” X.Smp.2.19; “καιροῦ μεῖζον” E.Fr.626 codd.;προσωτέρω or πορρωτέρω τοῦ κ., X.An.4.3.34, HG7.5.13; ὀξύτερα τοῦ κ. Pl.Plt.307b; νωθεστέρα τοῦ κ. ib.310e;ὑπερβάλλων τῇ φιλοτιμίᾳ τὸν κ. Plu.Ages.8, cf. Hp.Loc.Hom.44.

II. of Place, vital part of the body (cf. “καίριος” 1), “ἐς καιρὸν τυπείς” E.Andr.1120.

III. more freq. of Time, exact or critical time, season, opportunity, Χρόνου κ. S.El. 1292: usu. alone, κ. [ἐστιν] ἐνᾧ Χρόνος οὐ πολὺς κτλ. Pl.Lg.709c; ἔχει κ. τι it happens in season, Th.1.42, etc.; κ. ἔχειν τοῦ εὖ οἰκεῖν to be the chief cause of. 

b. adverbial phrases, ἐς καιρόν in season, Hdt. 7.144, E.Tr.744, etc.; (but also οἱ κατὰ κ. ἡγεμόνες in office at the time, Pl. Plt.277a;πρὸ καιροῦ prematurely, A.Ag.365 (anap.); ἐπὶ καιροῦ also means on the spur of the moment, “ἐπὶκ. λέγειν” Plu.Dem.8, cf. Art.5; “ἐξενεγκεῖν πόλεμον” Id.Ant.6. 

2. season, πᾶσιν καιροῖς at all seasons of the year, IG14.1018, cf. LXX Ge.1.14, Ph.1.13, Porph. ap.Eus.PE3.11; κ. ἔτους, later Gr. for Att. ὥρα ἔτους, acc. to Moer.424; time of day, Philostr.VA6.14.
b. critical times, periodic states, “καιροὶ σωμάτων” Arist.Pol.1335a41.
3. generally, time, period, “κατὰ τὸν κ. τοῦτον” Plb.27.1.7; “κατ᾽ ἐκεῖνο καιροῦ” Conon 3, al.: more freq. in pl., κατὰ τούτους τοὺς κ. Arist.Ath.23.2, al., cf. Plb.2.39.1; τὰ κατὰ καιρούς chronological sequence of events. 

4. in pl., οἱ καιροί the times, i. e. the state of affairs, freq. in bad sense, ἐν τοῖς μεγίστοις κ. at the most critical times, X.HG6.5.33, cf. D.20.44; “περιστάντων τῇ πόλει κ. δυσκόλων” IG22.682.33, etc.: also in sg., X.An.3.1.44, D.17.9; ὁ ἔσχατος κ. extreme danger, Plb.29.27.12, etc.; “καιρῷ δουλεύειν” AP9.441(Pall.).

IV. advantage, profit, τινος of or from a thing, Pi.O.2.54, P.1.57; εἴ τοι ἐς κ. ἔσται ταῦτα τελεόμενα to his advantage, Hdt.1.206; ἐπὶ σῷ κ. S.Ph.151 (lyr.); τίνα κ. με διδάσκεις; A.Supp.1060 (lyr.); τί σοι καιρὸς . .καταλείβειν; what avails it.? E.Andr.131 (lyr.); τίνος εἵνεκα καιροῦ; D.23.182; οὗ κ. εἴη where it was convenient or advantageous, Th.4.54; ᾗ κ. ἦν ib.90; Χωρίον μετὰ μεγίστων κ. οἰκειοῦταί τε καὶ πολεμοῦται with the greatest odds, the most critical results, Id.1.36.