cgs is the system of units based on measuring lengths in centimeters, mass in grams, and time in seconds. It is a metric system, although not the flavor of the metric system used most commonly. It was introduced by the British Association for the Advancement of Science in 1874, and was immediately adopted by many working scientists.
There are several flavors of the cgs system: "cgs electrostatic," "cgs electromagnetic," and "cgs Gaussian." None of these are part of the SI system, except for units such as the centimeter defined in both systems (Taylor 1995, p. 11). The cgs Gaussian system is nonetheless commonly used in theoretical physics, while the MKS system (based on the meter, kilogram, and second) is commonly used in engineering and physics instruction.
The following table summarizes common quantities and their units in both MKS and cgs.
References
Jackson, J. D. "Conversion of Equations and Amounts between SI Units and Gaussian Units." Appendix §4 in Classical Electrodynamics, 3rd ed. New York: Wiley, pp. 783-784, 1998.
Purcell, E. M. Backmatter in Electricity and Magnetism, 2nd ed. New York: McGraw-Hill, 1985.
Rowlett, R. "CGS and MKS Units." http://www.unc.edu/~rowlett/units/cgsmks.html.
Taylor, B. N. "CGS units." §5.3.1 in "Guide for the Use of the International System of Units (SI)." NIST Special Publication 811, p. 11, 1995 Edition. http://physics.nist.gov/Document/sp811.pdf.
