This is the 17183rd most frequent Russian word.
столетий
"Centuries" (genitive plural of "век" - century).
In this sentence, "столетий" is used as the plural genitive form referring to centuries (literally 'of centuries'), indicating a span of time exceeding three centuries.
За последние три столетия мир сильно изменился.
Over the last three centuries, the world has changed significantly.
Here, "столетий" is also in the plural genitive form, describing styles or attributes associated with multiple centuries from the past.
Архитектура этого города сохранена в стиле прошлых столетий.
The architecture of this city is preserved in the style of past centuries.
In this sentence, "столетий" is used in the plural genitive form to denote a duration of time spanning several centuries.