Década vs. decénio vs. decênio
While década, decénio, and decênio all refer to a period of ten years, the distinct differences lie in their frequency of usage and regional spelling rules. Década is the universal term, while decénio and decênio are regional spelling variations of the same word.
Década
B1This is the most common and versatile term used in all Portuguese-speaking countries. It is appropriate for both formal and informal contexts and is the standard choice when referring to specific historical eras, such as the 60s or the 90s.
A moda da década de oitenta era muito colorida.
(The fashion of the eighties was very colorful.)
Já se passou uma década desde que me mudei para esta cidade.
(It has already been a decade since I moved to this city.)
Esta foi a década mais quente da história.
(This was the hottest decade in history.)
Decénio
C1This word specifies a period of ten years and is primarily used in European Portuguese (Portugal) and African Portuguese-speaking countries. It is spelled with an acute accent (´) and is often found in slightly more formal calling or literature compared to década.
O último decénio trouxe grandes avanços na medicina.
(The last decade brought great advances in medicine.)
O plano estratégico cobre o próximo decénio.
(The strategic plan covers the next decade.)
Ele dedicou um decénio da sua vida a este projeto.
(He dedicated a decade of his life to this project.)
Decénio
C1This is the Brazilian Portuguese spelling of the previous word. In Brazil, it takes a circumflex accent (^) due to differences in pronunciation (a more nasal vowel sound). Like its European counterpart, it is less common than década and appears more often in formal writing.
A empresa comemorou seu primeiro decênio de existência.
(The company celebrated its first decade of existence.)
Este decênio será crucial para a preservação ambiental.
(This decade will be crucial for environmental preservation.)
O tribunal analisou casos do último decênio.
(The court analyzed cases from the last decade.)
Summary
In short, use década for general conversation and historical eras anywhere in the world. If you want a synonym for variety in formal writing, use decénio if you are writing for Portugal or Africa, and decênio if you are writing for Brazil.