Silinvar is Patek Philippe's proprietary silicon alloy. The name refers to “silicium” (German for “silicon”) and “invariable”. The latter is a reference to the “Invar” spring material invented in the 19th century and used by Patek Philippe ever since.
Patek Philippe debuted Silinvar in 2005 in their Ref. 5250 Advanced Research watch, which used the material in the escape wheel.