The word litre is derived from an older French unit, the litron, whose name came from Greek via Latin. The original French metric system used the litre as a base unit.

The spelling of the word used by the International Bureau of Weights and Measures is "litre". This spelling is also the usual one in English in most English-speaking countries, but the most common American English spelling is "liter", which is officially endorsed by the United States.

A litre is defined as a special name for a cubic decimetre (1 L = 1 dm3 = 103 cm3). Hence 1 L ≡ 0.001 m3 (exactly). So 1,000 L = 1 m3

From 1901 to 1964 the litre was defined as the volume of one kilogram of pure water at 4 °C and 760 millimetres of mercury pressure. During this time, a litre was about 1.000028 dm3. In 1964 this definition was abandoned in favour of the current one.

Litres are most commonly used for items (such as fluids and berries), which are measured by the capacity or size of their container, whereas cubic metres (and derived units) are most commonly used for items measured either by their dimensions or their displacements. The litre is often also used in some calculated measurements, such as density (kg/L), allowing an easy comparison with the density of water.

One litre of water has a mass of almost exactly one kilogram when measured at its maximal density, which occurs at about 4 degrees Celsius. Similarly: 1 millilitre of water has a mass of about 1 g; 1,000 litres of water has about 1000 kg of mass. This relationship holds because the gram was originally defined as the mass of 1 mL of water; however, this definition was abandoned in 1799 because the density of water changes with temperature and, very slightly, with pressure.

The litre, though not an official SI unit, may be used with SI prefixes. The most commonly used derived unit is the millilitre, defined as one-thousandth of a litre, and also often referred to by the SI derived unit name "cubic centimetre". It is a commonly used measure, especially in medicine and cooking. Other units may be found in the table below, where the more often used terms are in bold. However, some authorities advise against some of them; for example, in the United States, NIST advocates using the millilitre or litre instead of the centilitre.

One litre is slightly more than one U.S. liquid quart and slightly less than one Imperial quart or one U.S. dry quart.

One measured cup ≈ 250 mL.

One teaspoon ≈ 5 mL (In some countries, this is an exact equivalency by definition of the teaspoon).