Ordinal decades are decades referenced by ordinal numbers like "first," "third," and "one-hundred-and-forty-fifth." This references their relative position in the sequence.

Ordinal decades don't directly have anything to do with the numbers in the year. However, since both decades and our number system's base are based on the number 10, they tend to line up. Notably though, they don't at the change of the decades. Since there was no year zero (it went from 1 BC to AD 1), the first decade was from 1–10. The second was from 11–20, and so on.

This means that these last years will be different between ordinal decades and cardinal decades. The years 1920 and 1921 were both in the 1920s' cardinal decade, but they were split between the in the 192nd and 193rd ordinal decades.

Practically, ordinal decades are not that meaningful. People usually just care what the first three digits are, not how many decades we've had in this calendar era. About all ordinal decades have going for them is that they do align with the more commonly-used ordinal centuries. The 21st century began at the exact same time as the 201st decade. Aside from that though, there's not a lot of use for them. Then again, grouping years by cardinal decades is still just as arbitrary, so it's not a huge deal what you use. The most important thing is to be clear what you are referring to so you can communicate clearly.