[morzh]

SI is System International.

http://en.wikipedia.org/wiki/Internation...m_of_Units

It is pretty much a standard system of units used in physics.
Not all physicists like SI, some still prefer CGS (centimeter-gram-second), which derivatives, CGSE (electrical) and CGSM (magnetic) are popular in electricity/magnetism books.
For example, a unit of force in SI is "Newton" (roughly 1/10 of a kilo) but in CGS it is "dyne" which is 1/100,000 of Newton.
or unit of energy: in SI it is a "Joule" and in CGS it is an "erg" which is 1/10,000,000 of "joule".