A nanogram is a unit of measurement, abbreviated as "ng" indicating a mass equal to one billionth or 1/1,000,000,000, of a gram. This is obviously an extremely small amount and nanograms as units are rarely used outside of the scientific fields of microbiology, physics, and chemistry. A nanogram is a unit derived from the International System of Units (SI), a system recognized by nearly every country around the world as the system by which consistent measurements may be made and communicated. The SI is closely related to and derived directly from the metric system.
Mass and weight are frequently confused, and the main reason for this is that at standard Earth gravity, the mass and the weight of an object can be regarded as identical, and the terms are often used interchangeably. The two are not the same however as mass is a measurement of the amount of matter in an object and weight is the measure of how strongly gravity pulls at an object. At less than standard Earth gravity, therefore, an object retains the same mass, but when weighed on a traditional scale, weighs less than at standard gravity. For this reason, units of measurement that express the mass of an object are regarded as being more useful to science. SI units of mass, such as the nanogram, are often mistakenly identified as units of weight and while this is functionally correct on the Earth's surface, it is not completely accurate.
In order to further understand the significance of the SI base unit of mass, the kilogram, and by extension, divisions and multiples of this unit, it is necessary to know where this unit has its origin. The kilogram is defined as the mass of 1,000 cubic centimeters of water. A gram therefore, is the mass of one cubic centimeter of water. A nanogram then, is a mass equivalent to one billionth of a cubic centimeter of water. This relationship can be cumbersome to express this way, so a nanogram is sometimes expressed in scientific notation as a factor of a gram, and as such is written as 10^{-9} grams.
In certain scientific fields, nanograms are often used as units of measurement. Extremely small amounts of matter may be measured in nanograms, and units this small may be encountered in some calculations in certain fields. Microbiologists may use nanograms to express the mass of extremely small biological units such as viruses. Modern drug testing techniques can detect drug concentrations on the order of nanograms in hair samples. These are just a few of the many instances in which nanograms may be used as units.