A lightyear (abbreviated ly) is a unit of distance, defined as the distance light can travel in one year. At the maximum* speed of light, approximately 300,000 kilometres per second (186,000 miles per second), this translates to approximately 9.461 trillion kilometers, or 5.879 trillion miles. By comparison, it takes light just 8.32 minutes to reach earth from our sun.
Somewhat incongruously, the year used for this measurement is called the Julian year, which is not to be confused with the Julian calendar year or the modern Gregorian calendar year or with any other way of measuring a calendar year. Calendar years are usually measured against the moon or the sun, while the astronomical Julian year is measured against the standard second, which in turn is measured against vibrations of the caesium atom. In astronomy, this period of time is considered to be equal to 365.25 days, each consisting of 86,400 standard seconds. While the Julian year is not one of the standard units of measurement sanctioned by the International System of Units (SI), it nevertheless provides a convenient measure of time within astronomical durations. For this reason, its use is encouraged by the International Astronomical Union.
(Julian days also exist as an astronomical calendar measure, but they have no relevance either to Julian years or to lightyears, and thus fall outside the scope of this article.)
Lightyears are divided or multipled in standard metric increments: kilo- (times one thousand), mega- (times one million) and giga- (times one American billion, or one thousand million). By way of a rough sense of scale, our nearest stellar neighbour, Proxima Centauri, is 4.22 lightyears away. Our galaxy, the Milky Way, is one hundred kilolightyears across. Our nearest galactic neighbour, the Andromeda galaxy, is 2.5 megalightyears away. Finally, the perceived edge of the visible universe is 46.5 gigalightyears from earth.
The official standard unit of distance in astronomy is the parsec, or parallex of one arcsecond. It is defined as the distance from earth to a stellar object which, observed from earth, has a parallex angle of one arcsecond. Parallex angle refers to the apparent difference in position of a stellar object when observed at opposite times of year, such that the difference in location of the earth is equal to two astronomical units. (One astronomical unit represents the average distance of the earth from the sun.) Arcsecond refers not to time but to angular measurement: 1/60 of an arcminute, which in turn is 1/60 of a degree. One lightyear is approximately the same distance as 0.3 parsecs. Conversely, 3.26 lightyears equals one parsec.
* Light can also move much, much slower than this; to the point where it has been measured at only a few metres per second. However, it cannot move faster within the known laws of our universe.