International System of Units
The International System of Units prescribes the symbols and prefixes shown in the table to form decimal multiples and submultiples of SI units.
The following examples illustrate the use of these prefixes:
0.000,001 meters = 1 micrometer = 1°m
1000 meters = 1 kilometer = 1 km
1,000,000 cycles per second = 1,000,000 hertz = 1 megahertz =1 MHz
Source: NASA (http://earthobservatory.nasa.gov/Glossary)