The concept of Decade: From the perspective of calendar units, "decade" is a calendar unit in ten years, the same is lustrum (5 years), century (100 years), millennium (1000 years). In the past decade synonyms 'in the past decade' means that it happened at some point within the last ten years. I guess the only minute difference would be how we use them.
Dolly Parton’s Iconic Hair Through the Years
In a decade = in 10 years time this has no specific date.
In the/this decade = in these 10 years this implies you're talking about the current decade.
I mean we can say two decades or three decades to. 'over the past decade' means that it happened repeatedly, more than once, or.
