Speaking of the Hellatin words . . .

In many scholarly works on chronometry etc I've run across the term "nycthemeron" for the 24-hour period of the Earth's rotation (to avoid the ambiguity inherent in the term "day").  It looks awfully Greek to me.  My questions:

1. is it well-formed?
2. what's the etymology?
3. what's the plural?

Thanks.