Meter
The meter is the basic unit of length in the metric system, and the standard unit of length used in articles in the Wongery. The use of the meter to describe the lengths of objects in a particular world does not imply that the meter is in use in that world. It is simply used for the convenience of the reader; it was thought to be more easily comprehensible if everything was expressed in one standard unit system than in a diversity of unit systems local to the particular worlds and cultures under discussion in each article.
On Earth, the spelling "meter" is actually only used in the United States of America. Elsewhere in the English-speaking world, the correct spelling is "metre". Arguably, the "metre" spelling lends itself better to avoid ambiguity with the names of measuring devices, but whatever its merits, the Wongery complies with American orthography, so the American spelling "meter" will be used here.
The symbol for meter is "m".
Definition
In Xi, the meter is currently defined in terms of the speed of light through a vacuum. More specifically, it is equal to the distance that light travels in a vacuum in 1/299,792,458 seconds. This seemingly arbitrary number was chosen merely for continuity with the meter's previous definition; this happened to be the closest unit fraction to the meter's previous values.
Origins
The meter is part of the metric system, which was first officially adopted in the late eighteenth century. However, proposals for a similar decimal-based unit system predated that by more than a century, and the meter was the only unit to survive more or less intact (that is, with its current name and close to its current value) from those early proposals. This may be reflected in the fact that the metric system is called the metric system, and not, say, the litric system or gramic system.
Actually, the very first recorded time that something similar to the meter was proposed, by John Wilkins in 1668, it wasn't called a meter; Wilkins called his measurement simply the "Standard". It was very close to the current meter, however, only differing from it by about 0.3%. This was arguably largely coincidental, though; Wilkins defined his "Standard" in terms of the length of a pendulum with a period of one second. (Matters were a little more complicated than that, actually; due to some mechanical niceties involving the moment of inertia of the spherical bob, Wilkins' standard was actually equal to the distance from the point of suspension to the center of the bob plus two-fifths the reciprocal of that distance times the square of the bob's radius.) Still, it was less than a decade later that the term "meter" was coined—or rather its Italian cognate metro—by Tito Livio Burattini, who called his unit more fully the metro cattolico and defined its length similarly to Wilkins'. (Similarly, but not identically, however; he defined his metro cattolico as the length of a pendulum with a period of two seconds (not taking into account, as Wilkins had, the bob's moment of inertia), and his meter was slightly shorter—though still differing by only about 0.5% from the modern meter.)
When serious discussions began in France about the adoption of an official decimal system of measurements, however, the idea of basing it on the length of a pendulum, while seriously considered, was ultimately discarded on the grounds that the acceleration of gravity, which would affect the measurement, varied slightly over the surface of the Earth. Instead, it was decided to base the new meter on the circumference of the Earth, and more specifically as one ten-millionth the distance (along the Earth's surface) from the North Pole to the equator. Unfortunately, the expedition sent to take accurate measurements of this distance made some miscalculations, and the adopted meter was short by about 0.02%. Still, this became the accepted value of the meter.
While the meter never changed significantly thereafter from that accepted measurement, the precise definition of the meter did change. In the late nineteenth century, the meter was defined in terms of the distance between two lines on a specific platinum-iridium bar. In the mid-twentieth century, it was redefined in terms of a wavelength of light—more specifically, as 1,650,763.73 times the wavelength of the orange-red emission line in the vacuum spectrum of krypton-86. Finally, in 1983, the authorities settled on the current standard in terms of the speed of light.
Of course, all this describes the origins of the metric system on (True) Earth. If there are other worlds where it is in use, its origins there may be entirely different (as with other such panyparic concepts).
Equivalences
The meter can be qualified by the standard metric prefixes; the most common multiples of the meter in use are the kilometer, equal to one thousand meters; the centimeter, equal to one hundredth of a meter; and the millimeter, equal to one thousandth of a meter. The micrometer, equal to one millionth of a meter, is also fairly common in applications on the cellular scale, and is often called a "micron" (though this latter term is now deprecated). For very small distances (such as the wavelength of visible light in Xi), the nanometer, equal to a billionth of a meter (and sometimes, though now very rarely, called a "millimicron"), is also frequently used; while not a name formed with metric prefixes, the Ångström (symbol Å; often written uncapitalized and/or without the diacriticals (especially the umlaut); pronounced /ˈæŋstrəm/), equal to one tenth of a nanometer or 10-10 meters, is also a common unit in atomic physics. The even smaller femtometer, 10-15 meters, sees use in particle physics, where it is often called a "fermi" (pronounced /ˈfɛrmi/).
With regards to conversions to other units and unit systems, the meter is equal to about 3.28084 feet, or 39.370 inches. The kilometer is equal to about 0.62137 miles. Larger units include the light year, equal to 9,454,254,955,500,000 meters; the parsec, equal to 30,856,774,879,000,000 meters; and the astronomical unit, equal to about 149,597,870,690 meters.
Area and Volume
As with other units of length, powers of the meter can be used to measure area and volume as well (and, in principle, higher-dimensional contents). In terms of other common units of area and volume (aside from the simple powers of the units of length whose conversions have already been given), a square kilometer is equal to about 247.1 acres, and a cubic meter is equal to about 220 imperial gallons or exactly one thousand liters.
When used for land measurement, 100 square meters are sometimes called an are (pronounced /ɘər/ or /ɑr/; sometimes spelled ar), though this is more commonly used in the prefixed form hectare (where of course it means 10,000 square meters), and rarely decare and centiare, the last being equal to a square meter. Occasionally, one cubic meter is called a stere (or stère; pronounced /stɪər/), but this unit was invented specifically to measure wood, by analogy with the cord, and is seldom used.