Abarim Publications' online Biblical Greek Dictionary
The noun καιρος (kairos) is a bit of a troubler because there's no proper equivalent in English, and that may be because it expresses something crucial to a pre-industrial society but not so relevant for technologically advanced moderns. It was so crucial that Qoheleth recorded a hymn devoted to it: timeliness, the right time or place for a certain event (Ecclesiastes 3:1-8).
In the days of Homer our word mostly denoted a right place (on the body, to molest so as to inflict specified encumbrances) but in later ages our word denoted mostly the right, proper or opportune time. The ancients had no clocks and watches, and regarded time not as an absolute stage upon which events unfolded but rather as a series of cycles (called χρονος, chronos) brought about by the interactions of all things. Particularly when mankind made the transition to agriculture, understanding the year and its precise times, periods and seasons became crucially important.
For these reasons, our noun καιρος (kairos) is thought to relate to the verb κειρω (keiro), to shear (to remove hair from sheep or human heads). This verb comes from the Proto-Indo-European root "(s)ker-", meaning to cut off (hence to harvest). From that same root comes the noun κοραξ (korax), raven, which explains its association to reaping and storing (Luke 12:24).
When in the New Testament this word καιρος (kairos) is used, it refers to a specific period in which an associated event can, will or must take place; a period not in the sense of any old stretch of time but a period specifically for an associated goings on. The way a pronoun takes the place of a noun in a discourse, our word καιρος (kairos) speaks of an opportune time for some particular event, which may not be mentioned but is thus referred to. It is used 86 times, see full concordance, and from it derive:
- Together with the preposition α (a), meaning without: the verb ακαιρεω (akaireo), literally meaning to be without a proper time or period; unscheduled, and thus without practical context. This word occurs in Philippians 4:10 only, but it's unclear whether Paul is being harsh, facetious or generous. This word is rare in the classics. In his Library, Diodorus described how, contrary to the followers of Pythagoras, certain people of his own time (1st century BC) shirked certain virtues and responsibilities by saying that those were "none of their business" (our word), whilst having ample time to meddle in other people's private affairs — "the result of it all being that they busy themselves when they have no business and show no concern when they are concerned" (Lib 10.7.3).
- Also together with the preposition α (a), meaning without: the adverb ακαιρος (akairos), also meaning without a proper time, or rather without a proper cue or lead; out of the blue or inappropriately. This word occurs in 2 Timothy 4:2 only, juxtaposed with the adverb ευκαιρος (eukairos); see below.
- Together with the adverb ευ (eu), meaning good: the adjective ευκαιρος (eukairos), meaning to be well-timed, spot on, well-appropriately (Mark 6:21 and Hebrews 4:16 only). From this word in turn derive:
- The verb ευκαιρεω (eukaireo), meaning to be convenient, appropriate or opportune, or quite literally: to have a good time, to spend one's free time (Mark 6:31, Acts 17:21 and 1 Corinthians 16:12 only).
- The noun ευκαιρια (eukairia), which describes the convenient, appropriate or opportune time to do something (Matthew 26:16 and Luke 22:6 only).
- The adverb ευκαιρος (eukairos), meaning well-timed, spot on, well-appropriately (Mark 14:11 and 2 Timothy 4:2 only).
- Together with the preposition προς (pros), which describes a motion toward: the adjective προσκαιρος (proskairos), which is a broadly applicable term that denotes the forming of a stretch of time. It can be translated with: lasting a while (implying an extended time), having a temporary validity (implying a limited time), occasional or extraordinary (in the sense of applying only to this specific time). This adjective is used 4 times; see full concordance.