Coss

The variable (or unknown) was called
name
For example by
Arabic say Al-Khwarizmi
al-Qalasadi (1400-1486) (he uses the first letter of "say" on the top of the corresponding coefficient)
Latin res Cardano
Italian cosa Cardano
German Coß
oder auch: Dingk 		Adam Ries
("Coß" is just a German version of "cosa".)
The German algebraists of the 15th and 16th century were named the "Cossisten".