das Jahrhundert
century
A period of one hundred years.
Das Automobil wurde Ende des 19. Jahrhunderts erfunden.
The automobile was invented at the end of the 19th century.

century
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century
A period of one hundred years.
Das Automobil wurde Ende des 19. Jahrhunderts erfunden.
The automobile was invented at the end of the 19th century.

The word is built very logically. It consists of 'Jahr' (year) and 'hundert' (hundred). So a 'Jahrhundert' (century) is literally a period of one hundred years.
Ein Jahrhundert hat zehn Jahrzehnte.
A century has ten decades.
To name a specific century, you use the preposition 'im' (in the), an ordinal number with a period, and the word 'Jahrhundert'. For example, 'im 21. Jahrhundert' (in the 21st century).
Viele wichtige Erfindungen stammen aus dem 19. Jahrhundert.
Many important inventions come from the 19th century.
To describe part of a century, the genitive is often used. Typical phrases are 'Anfang des Jahrhunderts' (at the beginning of the century), 'Mitte des Jahrhunderts' (in the middle of the century), and 'Ende des Jahrhunderts' (at the end of the century).
Meine Urgroßeltern wurden Anfang des 20. Jahrhunderts geboren.
My great-grandparents were born at the beginning of the 20th century.
decade
'das Jahrhundert' and 'das Jahrzehnt' both refer to a period of time, but the length is very different. A 'Jahrhundert' has one hundred years; a 'Jahrzehnt' has ten years. Learners may confuse the words because both begin with 'Jahr-' and are used for history or time references. Memory tip: A decade is short compared with a century; ten decades make one century.

Dieses Schloss wurde im 18. Jahrhundert gebaut.
This castle was built in the 18th century.
In den letzten zehn Jahren hat sich die Stadt stark verändert.
In the last ten years, the city has changed a lot.