This is the 157th most frequent Catalan word.
rus
"Rus" in Catalan means "Russian" (adjective or noun) or refers to the "Russian language."
Here, 'rus' (as 'russa') is used as an adjective describing the type of history, specifically related to Russia.
El llibre sobre història russa és fascinant.
The book about Russian history is fascinating.
In this usage, 'rus' (as 'russos') is a noun used to refer to the people of Russia, the Russians.
Els russos celebren el Nadal el 7 de gener segons el calendari julià.
The Russians celebrate Christmas on January 7th according to the Julian calendar.
Here, 'rus' is used as an adjective indicating the nationality of the student.