The Système Internationale d’Unités (SI), or International System of Units, defines seven units of measure as a basic set from which all other SI units are derived. The SI base units and their physical quantities are:
The SI base quantities form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science and technology. However, in a given realization in these units they may well be interdependent, i.e. defined in terms of each other.
An order of magnitude is the magnitude, or scale, of a class of numbers such that the numbers in that class have a fixed ratio to the class preceding it.
In the most common usage of the term "order of magnitude," the fixed ratio is 10 and the scale is the base-10 exponent that is applied to 10. Therefore, to be an order of magnitude greater means to be 10 times as large.
A gram per liter or litre (g/L) is a unit of measurement of mass concentration that shows how many grams of a certain substance are present in one litre of a usually liquid or gaseous mixture. It is not an SI unit because it contains the non-SI unit litre. The SI unit of mass concentration is kilogram per cubic meter, which is numerically equivalent (1 g/L = 1 kg/m3).
SI-style prefixes are often applied, giving units like milligrams per litre (mg/L) or micrograms per decilitre (µg/dL). When measuring concentration in water, parts per million is an older expression of mg/L, since one liter of water under standard conditions weighs one kilogram or one million milligrams.
In physics, mass (from Greek μᾶζα "barley cake, lump [of dough]") is a property of a physical system or body, giving rise to the phenomena of the body's resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies. Instruments such as mass balances or scales use those phenomena to measure mass. The SI unit of mass is the kilogram (kg).
For everyday objects and energies well-described by Newtonian physics, mass has also been said to represent an amount of matter, but this view breaks down, for example, at very high speeds or for subatomic particles. Holding true more generally, any body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction; the term matter has no universally-agreed definition under this modern view.