(a) the period in which the earth turns once on its axis; = day, n. 2a; (b) the part of this period when the sun is up; = day, n. 1 (rare).
A day. Often with preceding ordinal number in expressions referring to a period of time consisting of a number of days; also more generally in…
a day of 24 hours beginning at noon (see astronomical day n. at astronomical, adj. n.compounds).
A solar day on the planet Mars (24 hours 39 minutes).