Milliliters are units of measurement used to measure the volume, or three-dimensional space, inhabited by an object. One liter is equal to 1,000 milliliters. The base word liter is often spelled differently based on geography; liter is the favored spelling in American English, while litre is used more often in European English. While the liter does not technically belong to the international system of measurement units, it is still commonly used in all branches of science. The standard unit of volume measurement is the cubic meter; one cubic meter is equal to 1,000 liters and one milliliter equals one cubic centimeter.
The milliliter is based on the liter, just as many other volume measurements are. The specific degree of the measurement depends on the prefix in front of "liter," which simply indicates that the value is a measure of volume. "Milli" means one one thousandth, so a milliliter is one one thousandth of a liter. This system of prefixes is used for many different measurements; one millimeter, for example, is one one thousandth of a meter. Though they are seldom used, other prefixes can be used to measure different degrees of volume; a microliter is one thousand times smaller than a millimeter.
One American teaspoon contains roughly five milliliters and one cup contains roughly 250 milliliters. Milliliters are extremely useful units of small but not microscopic measurements; many ingredients in cooking are measured in milliliters as are many substances in chemistry laboratories. Milliliters are generally not useful when measuring any object or substance that has a volume of more than one liter; it is much more difficult to understand the amount of space taken up by 2,500 milliliters than it is to understand the space taken up by two and a half liters.
At one point, one liter was defined as the volume taken up by one kilogram of water. This definition is not often used anymore, as the volume taken up by water varies significantly based on various factors such as temperature. There is, however, still a close relationship between the two systems of measurement; mass can be closely approximated from volume. One liter of water does, indeed, have a mass almost precisely equal to one kilogram. Similarly, simple conversions reveal that one milliliter of water has a mass that is roughly equal to one gram.