a year: a full twelve-month period, thought of as a complete annual cycle
This is simply the Greek word for a year. Across the New Testament it marks off a stretch of time about twelve months long, whether Paul staying a year and a half in Corinth, the church at Antioch meeting for a whole year, or the drought lasting three years and six months.
Older Greek kept a fine distinction between this word, which pictured the year as a completed round that comes back again, and ἔτος, which counted the year as a plain unit of time. By the first century that line had mostly faded and the two words overlap. The cyclical flavor still shows through, though, in the letter to the Hebrews, where the priest goes in "once a year" and offers "year by year" the same sacrifices that can never finish the work.
A few uses carry extra weight from their setting rather than from the word itself. When Jesus reads "the year of the Lord's favor" from Isaiah, the word for year points to the Jubilee, the appointed time of release; and when John repeatedly calls Caiaphas high priest "that year," he is naming the fateful year of the crucifixion, not suggesting the office changed hands every twelve months.