Década vs. decenio
While both década and decenio translate to decade in English, they are not always interchangeable in Spanish. The main difference lies in their frequency of use and specific connotations, with década being far more common and specifically used for calendar decades.
Década
A2A década is a period of ten years. It is the most common word for decade and is primarily used to refer to the specific decades of a century, such as the seventies, eighties, or nineties. It can also be used more generally to describe any ten-year span.
La moda de la década de los noventa está volviendo.
(The fashion of the nineties decade is coming back.)
Mi abuelo nació en la década de 1940.
(My grandfather was born in the 1940s.)
La compañía ha experimentado un crecimiento sin precedentes en la última década.
(The company has experienced unprecedented growth in the last decade.)
Se mudó a España y vivió allí por una década.
(He moved to Spain and lived there for a decade.)
La próxima década traerá muchos cambios tecnológicos.
(The next decade will bring many technological changes.)
Fue la mejor década de mi vida.
(It was the best decade of my life.)
Decenio
B2A decenio also means a period of ten years. However, it is a much less common and more formal or technical term. It is not typically used to refer to calendar decades (you would not say el decenio de los ochenta). Instead, it is often used in official, academic, or governmental contexts, sometimes for officially designated ten-year periods.
La ONU declaró el Decenio de la Agricultura Familiar.
(The UN declared the Decade of Family Farming.)
El proyecto de infraestructura se completará en el transcurso de un decenio.
(The infrastructure project will be completed over the course of a ten-year period.)
El primer decenio del siglo XXI estuvo marcado por la crisis económica.
(The first ten-year period of the 21st century was marked by the economic crisis.)
El informe analiza las políticas educativas del último decenio.
(The report analyzes the educational policies of the last ten-year period.)
Fue un decenio de agitación política y social en la región.
(It was a ten-year period of political and social unrest in the region.)
Summary
In summary, use década for almost all situations, especially when referring to calendar decades like the 80s or 90s. It is the common, everyday word. Reserve decenio for more formal, technical, or institutional contexts when referring to a generic or officially designated ten-year period. When in doubt, década is almost always the correct and more natural choice.