Year
In astronomy, the Julian year is a unit of time; it is defined as 365.25 days of exactly 86,400 seconds (SI base unit), totalling exactly 31,557,600 seconds in the Julian astronomical year.
Prophetic Year
Examples of use of the first definition:
Now this seventieth week is admittedly a period of seven years, and half of this period is three times described as "a time, times, and half a time," or "the dividing of times'"* twice as forty-two months;† and twice as 1,260 days.‡ But 1,260 days are exactly equal to forty-two months of thirty days, or three and a half years of 360 days, whereas three and a half Julian years contain 1,278 days. It follows therefore that the prophetic year is not the Julian year, but the ancient year of 360 days.§
(*) Dan. vii. 25; xii. 7; Rev. xii. 14.
† Rev. xi. 2' xiii. 5.
‡ Rev. xi. 3' xii. 6.
§ It is noteworthy that the prophecy was given at Babylon, and the Babylonian year consisted of twelve months of thirty days. That the prophetic year is not the ordinary year is no new discovery. It was noticed sixteen centuries ago by Julias Africanus in his "Chronography", wherein he explains the seventy weeks to be weeks of "Jewish" (lunar) years, beginning with the twentieth of Artaxerxes, the fourth year of the 83rd Olympiad, and ending in the second year of the 202nd Olympiad; 475 Julian years being equal to 490 lunar years.
The second and third definition can be said to follow from application of the day for a year principle.