CGS

CGS indicates the centimeter-gram-second system, which is a variant of the metric system developed for physics. Physics (and astrophysics) now use a different system, SI, which is adopted as an international standard, and is also a variant of the metric system, but physics and astrophysics also continue to use CGS. CGS treats those three units (centimeter, gram, and second) as basic, basing its other units on them. As far as I know, CGS, as used today, uses the SI-defined quantities for these, e.g., the CGS's centimeter is a hundredth of a standard SI meter. Given that, some of the CGS-specific units are thus merely alternate units to those of SI: for example, CGS's erg is merely a smaller unit of energy than SI's joule, but a quantity cited in either can be exactly converted to a quantity cited in the other. The continued use of these CGS units in astrophysics could be considered habit, but doing so does maintain consistency with past astrophysical literature.

However, other units of the two systems are defined differently, and a CGS-variant of electromagnetic units, termed the Gaussian system has units defined such that many differ from the analogous SI units by physical constant factors. A motivation for this CGS-variant's continued use is that some users find the Gaussian units more logical/natural for their purposes.