This is the 8237th most frequent German word.
jahrtausenden
"Jahrtausenden" means "millennia" in English.
In this sentence, 'jahrtausenden' refers to the timespan of thousands of years during which the pyramids have existed till now.
Die Pyramiden von Gizeh stehen seit Jahrtausenden.
The Pyramids of Giza have stood for millennia.
Here, 'jahrtausenden' implies many periods of a thousand years throughout the history of humanity.
In den Jahrtausenden der Menschheitsgeschichte gab es viele bedeutende Erfindungen.
In the millennia of human history, there have been many important inventions.
In this context, 'jahrtausenden' indicates the span of multiple thousands of years across which knowledge has developed.