litre

[F.]
Same as Liter. [Chiefly Brit.]

volume. Symbol l, since 1960 also L. (Metric) Perhaps the most familiar and most used but the most problematical of the metric units, the litre is simultaneously very convenient yet utterly superfluous, since volume can be expressed in terms of the cube of the linear unit. In fact it was not in the original metric system of 1791, being introduced two years later, as a sop to public demand and (with the are for area) the first example of official concession in the fight to establish what we now call metrication. Initially it was named the pinte, the cadil, and the litron (oddities in the context of post-revolutionary France, being relics from monarchist days, while the other units had been accorded original names), before acquiring its now-familiar name two years later. Of more concern to measurement, the litre has been ambiguous in its size, albeit only very modestly so.

The litre was conceived as 1 dm³ and identically the volume of 1 kg of water, since the gram was defined as the mass of 1 cm³ of water. However, following the creation in 1799 of the physical metallic-block prototype kilogram to avoid the use of the water standard, and as accurate measuring became more precise, it was discovered that the volume of a kilogram of water (at its specified maximum volumic mass and under normal atmospheric pressure) as measured by the prototype was discrepant from the intended 1 dm³. Construction of a revised prototype kilogram to fit the litre (hence the extant definition of the gram) was considered but rejected in 1872. Instead, the international committee asserted the 1799 prototype as defining the kilogram.
[Stott V. Nature Vol. 124, 622-3 (1929)] This re-sized the kilogram, but no change was made to the litre, hence entrenching the discrepancy. Careful measurement in 1901 gave the mass of 1 dm³ as only 0.999 970 7~ kg.
[Nature Vol. 65, 538 (1902)] The first official reaction to this discrepancy was to continue to leave the litre unchanged; thus, effective from 1901, it was accepted as 1.000 029~ dm³, making 1 mL not equal to 1 cm³ or ‘1 cc’ (and the litre not coherent as it had been ostensibly in the rarely used d.k.s. system). (Later measurements gave marginally smaller figures;
[Stott V. Proc. Roy. Soc. London Ser. A Vol. 186, 200-4 (1946)] see also below.)

The 1901 definition was formally abrogated in 1964, with the world congress asserting that the litre be regarded as a special name for, and hence identically the intended 1 dm³; simultaneously, though, it recommended the unit not be used in high-accuracy contexts. The litre is not part of the SI, though it is admitted for use with it. It is now normally regarded as precisely 1 dm³ for most scientific use, but presents problems relative to extant standard volumetric vessels. Working to five significant figures, the discrepancy affects only a minority of values, and them only to one unit in the last place; hence it can be effectively ignored in such a context. Where greater precision is intended, use of the unambiguous term dm³ (or similar) is appropriate and recommended, though scientific practice probably points to a complete equating of the two.

The final problem with litre concerns its abbreviation; officially it has long been the lower-case initial. However, since this character is often not clearly distinguishable from the unit digit, that official form is unsatisfactory for many documents. The upper-case initial is the simplest and commonest solution, but a switch to italics or a different (e.g. cursive) face is an alternative that can meet the lower-case policy. International standards allowed the capital as of 1979, but deprecate the use of italics or alternative faces. The capital form must be expected to prevail. (The litre not being a base unit, there is nothing inconsistent with the use of a capital rather than a lower-case letter.)

A series of coats of arms of the pious founders of certain churches in the Middle Ages and the Renaissance.

A unit of volume formerly defined as the volume occupied by a mass of 1 kg of pure water at its maximum density and standard atmospheric pressure. It is equal to 1.000028 dm³. It has subsequently been defined as a special name for a decimetre cubed. This has caused some confusion.

The noun has one meaning:

Meaning #1: a metric unit of capacity equal to the volume of 1 kilogram of pure water at 4 degrees centigrade and 760 mm of mercury (or approximately 1.76 pints)
  Synonyms: liter, l, cubic decimeter, cubic decimetre