Hypaethral

Hypaethral (pronounced hi-pee-thruhl, hahy-pee-thruhl)

In classical architecture, wholly or partly open to the sky.

Circa 15 BC: From the Classical Latin hypaethrus from the Ancient Greek ὕπαιθρος (húpaithros or hýpaithros) (open to the sky), from ὑπό (hupó), combining form of ὑπό (hupó) (under) + αἰθήρ (aithḗr) (air, ether).  The construct was hyp- (under) + aîthros (clear sky; see ether) + -al (from the Latin adjective suffix -ālis). 

The Roman architect Vitruvius, in his treatise De architectura (circa 15 BC) for the emperor Caesar Augustus (63 BC–AD 14 (also known as Octavian); first Roman emperor, 27-AD 14), used the Latin hypaethrus to describe temples in which the cella (the part of the temple housing an image of the deity) was wholly or partially uncovered.  In the late eighteenth century, English classicists adopted the re-modelled form hypaethral in their works about ancient architecture and another adjective they would employ was cleithral, designating temples with roofed central spaces; cleithral comes from the Ancient Greek kleithra (lattice).

The more modern American spelling is hypethral which probably is preferable but, unlike many US spellings which have made inroads elsewhere in the English speaking world, hypaethral remains the standard form; this is doubtless because it’s a technical word used by tiny subsets of historians, classists and architects.  Both spellings remain technically correct but use within any document needs to be consistent.

Hypaethral Temple at Paestum, Wilkins William, from The Antiquities of Magna Graecia, London, Longman, Hurst, Orme and Rees, MDCCCVII (1807).