nychthemeron

1. (uncommon) A period of one day and one night, a date: in the West, a period of 24 consecutive hours.Examples: "It has not yet been found by Observation, whether Mercury turns upon its own Axis, ... that is, whether it has the vicissitude of Day and Night, much less what is the space of its Nychthemeron or Natural Day; but one may safely enough conjecture that it does revolve about its Axis, since the other Planets do."; "Ideler passes over the Indians, and with good reason ; for they had not the week, and could not have had it, since they divided the nychthemeron into thirty hours, which the following out the same method would give rise to an entirely different series of planetary regents."; "Now, the Arabs assumed as the beginning of their Nychthemeron the point where the setting sun intersects the circle of the horizon. Therefore their Nychthemeron extends from the moment when the sun disappears from the horizon till his disappearance on the following day."Synonyms: twenty-four hour day, 24-hour dayuncommon