SI system and Si

Si and SI system are semantically related. In some cases you can use "Si" instead a noun phrase "SI system".

Si noun - A tetravalent nonmetallic element; next to oxygen it is the most abundant element in the earth's crust; occurs in clay and feldspar and granite and quartz and sand; used as a semiconductor in transistors.

SI system and si are semantically related. You can use "SI system" instead a noun "Si".