Zeitgeist

Zeitgeist (pronounced tsahyt-gahyst)

A German noun, the spirit of the time; general trend of thought or feeling characteristic of a particular period of time, historically especially as reflected in literature and philosophy although now also used to reference popular culture.

1835: From the German, Zeit + Geist, literally "time spirit (or ghost)", a calque of Latin genius sēculī, and best translated as “spirit of the age”.  It’s not commonly pluralized but the plural of Geist (ghost; spirit) is Geister, thus in English the irregular noun zeitgeister (also often in the plural), sometimes used of those who write of the fads in contemporary culture. Zeitgeist is a noun and zeitgeisty, zeitgeistier & zeitgeistiest are adjectives; the (rare) plural is zeitgeists.  Hopefully, zeitgeistesque never becomes a thing.  

Spirit of the age

A concept from eighteenth & nineteenth century German philosophy, best translated as "spirit of the age", it refers to the invisible forces dominating or defining the characteristics of a given epoch in history.  Although now most associated with German philosopher Georg Wilhelm Friedrich Hegel (1770-1831), especially in contrast with the Hegelian concepts of volksgeist (national spirit) and weltgeist (world-spirit), the coinage predates Hegel and appears in the work of Johann Gottfried Herder (1744–1803), Johann Wolfgang von Goethe ("Goethe"; 1749–1832), Herbert Spencer (1820–1903) and Voltaire (François-Marie Arouet; 1694–1778).  Hegel, in Phenomenology of the Spirit (1807), did write of the idea but preferred the phrase Geist der Zeiten (spirit of the times) over the compound Zeitgeist.  Most sources acknowledge the first documented use being in the writings of Herder.

Portrait of GWF Hegel (circa 1839), steel engraving by Lazarus Gottlieb Sichling (1812–1863) after an aquarel (watercolor) lithograph (1828) by Julius Ludwig Sebbers (circa 1785-1893).

Until recently, zeitgeist tended to be used retrospectively, in the manner of geochronology where an epoch has a known end date before labels are applied.  Of late, with its (perhaps over-enthusiastic) adoption by those who write of pop-culture, it’s come to be attached to just about anything, however fleeting.  Some criticize this but that's probably intellectual snobbery; in a sense Hegel et al were also writing of a type of popular culture.  Appropriately then, zeitgeist should now be considered and English word, having been thoroughly assimilated and not capitalized unless used in a way which references its continental origins (of Hegel, Voltaire etc) in which case it remains German and as a noun picks up an initial capital.  Because it remains both a German and English word, it can be spoken as written or translated, depending on the effect desired and in this it's a variant of the conventions which guide the way written text is handled in oral speech.  Where a word or phrase, however familiar in English, remains foreign it should when spoken, be rendered in translation: the written text “Hillary Clinton is, inter alia, crooked”, is spoken as “Hillary Clinton is, among other things, crooked”.  Where a foreign word or phrase has been assimilated into English it is treated as native so the written text “Hillary Clinton’s statement was the usual mix of lies, half-truths, evasions etc” is spoken as “Hillary Clinton’s statement was the usual mix of lies, half-truths, evasions etcetera.”  Note the usual shortened form (etc) has traditionally always been followed by a full-stop but there is a welcome revisionist movement which argues it too has become an English word (as etcetera is an anglicized form of the Latin et cetera) and thus needs no longer to be treated as a truncation.

Of the zeitgeist, early in the third millennium: Paramount Pictures promotional poster for Mean Girls (2004).

Where a foreign word or phrase, however familiar in English, depends for technical or other reasons on the original form to convey its meaning, it should be spoken as written.  Words of this class are often legal Latin such as obiter dictum (from the Latin and literally "something said in passing and not critically to what's being discussed" and in law describing a judge's expression of opinion not essential to the verdict and thus not binding as a precedent) and habeas corpus (from the Latin habeas corpus ad subjiciendum (literally "You (shall) have the body to be subjected to (examination)" and now a mechanism to challenge the lawfulness of a detention (ie the detainee must be brought before a court).  Status quo (from the Latin status (state) (sometimes used in the ablative statū) + quō (in which), the ablative of quī (which)) is well-known and widely used as kind of verbal shorthand to avoid clumsy English constructions yet the Status Quo is an Ottoman era firman (from the Ottoman Turkish فرمان‎ (ferman), from the Persian فرمان‎ (farmân) (command, order, decree)) which defines certain unchanging understandings among religious communities with respect to nine shared religious sites in Jerusalem and Bethlehem and to translate this to anything else would rob it of the meaning which relies on its historic context.  So, words evolve to be defined as assimilated into English or not according to "rules" which are a bit vague but in use there's probably a consensus things like "obiter dictum" and "habeas corpus" remain ways of expressing something with a foreign phrase because they're still used in their original (legal) context whereas "status quo" has become an English phrase because use is so diverse and distant from its origins.