This week’s interesting word is a borrowing from Greek. A nycthemeron is a period of twenty-four hours – one day and one night.
It can also be spelt nychthemeron. It is a term that seems to be used predominantly in academic texts, but I think it would fit in nicely in works of a more fantastical nature.
The OED gives the earliest recorded usage as follows:
“Onely the shadowy Vale of the Night will be cast over them once in a Nycthemeron.”
– Henry More, Two choice and useful treatises. 1682
I have taken this explanation of the origin directly from Oxford Dictionaries Online because Greek language is not my speciality:
“From Hellenistic Greek νυχθήμερον period of a day and a night, use as noun of neuter singular of νυχθήμερος lasting for a day and a night from ancient Greek νυκτ-, νύξ night + ἡμέρα day.” (You can view the entry here.)