"comprising a night and a day," 1690s, from Latin noct-, stem of nox "night" (see noct-) + dies "day" (see diurnal).
Entries linking to noctidial
noct-
also nocti-, word-forming element meaning "night, by night, at night," from Latin nox (genitive noctis) "night," from PIE *nekwt- "night" (see night).
diurnal adj.
late 14c., "daily, happening every day," from Late Latin diurnalis "daily," from Latin dies "day" + -urnus, an adjectival suffix denoting time (compare hibernus "wintery"). Dies "day" is from PIE root *dyeu- "to shine" (source also of Sanskrit diva "by day," Welsh diw, Breton deiz "day;" Armenian tiw; Lithuanian diena; Old Church Slavonic dini, Polish dzień, Russian den).
From early 15c. as "performed in or occupying one day;" 1620s as "of or belonging to the daytime (as distinguished from nocturnal). Related: Diurnally.