This is the 2541st most frequent Russian word.
рубеж
Boundary; frontier; milestone.
In this sentence, 'рубеж' is used metaphorically to represent the transition point between centuries.
На рубеже веков произошли значительные изменения.
At the turn of the century, significant changes occurred.
Here, 'рубеж' literally means a boundary or line of division between territories.
Граница реки служила рубежом между двумя государствами.
The river's border served as a boundary between two states.
In this context, 'рубеж' refers to a milestone or a significant point of achievement within a process.