A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit.
While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well.
However, megagram (and gigagram, teragram, etc.) are rarely used; tonnes (and kilotonnes, megatonnes, etc – although these units generally are not used as a measure of mass per se, but rather TNT energy equivalent of a mass) or scientific notation are used instead.
Megagram is occasionally used to disambiguate the metric tonne from the various non-metric tons.
Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six dating back to the system's introduction in the 1790s.Unit prefixes for amounts that are much larger or smaller than those actually encountered are seldom used.In use, the kilogram, gram, milligram, microgram, and smaller are fairly common.(The latter two items include prefixes corresponding to an exponent that is not divisible by three.) Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre).For scientific purposes, the cubic metre is usually used.