A period of 10 years.
The years 2020, 2021, 2022, 2023, 2024, 2025, 2026, 2027, 2028 and 2029 are together one decade.
Here is a VW Camper Van from the 1960s (the decade 1960 to 1969):

A period of 10 years.
The years 2020, 2021, 2022, 2023, 2024, 2025, 2026, 2027, 2028 and 2029 are together one decade.
Here is a VW Camper Van from the 1960s (the decade 1960 to 1969):