(nik / THEE / me / ron) - n. The span of a full twenty-four hours, the daily rotational period of the earth.

Despite the unusual (to English-speakers) "chth" in the middle of this word, it becomes quite easy to pronouce and understand, once split into "nycht-," night, and "-hemeron," day. The original Greek is nykhthemeros, "lasting for a night and a day," from nykh, nykt-, "night" and (h)emera, "day."

Many languages have separate words for the two meanings of "day": "the daylight hours" and "the whole 24 hours." Russian, for example, has den' and sutki for those two meanings, and both are used frequently. In English, we use "day" and "daytime" - similar, but English-speakers know the difference.

While researching this word , I found that it is much beloved by obscure-vocabulary websites. ("learn a new word every nychthemeron!") I found it on an Estonian site listing prices for hotel rooms ("250 EEK/nychthemeron"). I also found it on several websites on parenting instruction - since babies do not operate on any set schedule, problems (sickness, hunger) may crop up at any point during the night or day, not just during normal waking hours. New parents are therefore often urged to observe their babies "throughout the nychthemeron."

It is, of course, quicker and simpler to simply say "night and day," so this word is on its way out. I did not expect to find this word in use at all on the web - but it is not really in use, as, every time I found the word, I also found its derivation, which struck me as sad. Repeated definition does not equal legitimate usage.

Related: nychthemeral, adj., "occurring in 24-hour periods."

Note: Sometimes spelled "Nycthemeron," as Webster 1913 has it listed, but that is incorrect.

David Grambs' The Endangered English Dictionary

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