This sounds strangely familiar... Oh, right, 'cause it's Greek for "ten-lot" (or "decade").
You can also use the native Slavic term "dziesięciolecie."
the was not given
So it means that "a" was.
Also you can just answer "decade".
I'm a bit curious. In Polish, does this word usually mean "10 years" (like in English), or "10 days" (like in Russian)?
10 years. I was very, very surprised to learn that technically it has other meanings (10 days/weeks/months), but I don't think I've ever seen those outside of a dictionary.