nychthemeron
A period of one day and one night, a date: in the West, a period of 24 consecutive hours.
uncommon
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.
Where it comes from
From Ancient Greek νυχθήμερον (nukhthḗmeron), noun use of the neuter singular form of νυχθήμερος (nukhthḗmeros, “lasting a day and night”), from νύξ (núx, “night”) + ἡμέρα (hēméra, “day”).
Adapted from the Wiktionary entry nychthemeron (CC BY-SA 4.0).