Origins
The word 'cathedral' conceals a piece of furniture. Strip away the ecclesiastical grandeur, the soaring Gothic arches, and the centuries of religious significance, and what remains is a chair — specifically, the bishop's chair.
'Cathedral' entered English in the thirteenth century from Anglo-Norman and Old French 'cathedrale,' which came from Late Latin 'cathedrālis ecclēsia,' meaning 'the church of the cathedra.' 'Cathedra' is a Latin borrowing of Ancient Greek 'καθέδρα' (kathedra), a compound of 'κατά' (kata, down) and 'ἕδρα' (hedra, seat). The literal meaning is 'a sitting down' or 'a thing to sit on.' In classical Greek and Roman usage, a kathedra was any chair, though it carried particular connotations of authority — professors sat in cathedrae to teach, judges in cathedrae to pronounce sentences.
The early Christian Church adopted the cathedra as a symbol of episcopal authority. The bishop's chair, placed in his principal church, represented his teaching authority and his jurisdiction over the diocese. A church that housed this chair was the 'ecclesia cathedralis' — the cathedral church. The distinction is crucial and frequently misunderstood: a cathedral is not defined by its size, its beauty, or its age. It is defined by the presence of the bishop's cathedra. The smallest, plainest church in the world, if it contains a bishop's throne, is technically a cathedral. The largest, most magnificent church, if it does not, is not.
Greek Origins
The Greek root 'ἕδρα' (hedra) has a productive life in English beyond 'cathedral.' It appears in geometry: a polyhedron is a solid with many faces (poly + hedra), a tetrahedron has four, a dodecahedron twelve. The connection is that 'hedra' means both 'seat' and 'base' or 'face' — the flat surface on which something sits.
The phrase 'ex cathedra' (from the chair) is used in Roman Catholic theology for papal pronouncements made with the full authority of the papal office. When the Pope speaks 'ex cathedra' on matters of faith and morals, the pronouncement is considered infallible under Catholic doctrine. The metaphor is that the Pope is speaking from his seat of authority — his cathedra.
Perhaps most surprisingly, the English word 'chair' itself is a descendant of 'cathedra.' The Latin word passed through Vulgar Latin into Old French as 'chaiere' (later 'chaire' and then 'chaise' in some dialects), and from there into Middle English as 'chaiere' or 'chaire,' eventually simplified to 'chair.' The transformation from 'cathedra' to 'chair' — with the loss of the initial syllable, the softening of consonants, and the reduction of the ending — is a textbook example of phonological erosion across centuries and languages.
Spelling and Pronunciation
The word 'chaise' — as in 'chaise longue' (long chair) — is another French descendant of the same root, from a dialectal pronunciation of 'chaire.' And 'chaise longue' has been further mangled in English folk etymology to 'chaise lounge,' reinterpreted as if it had something to do with lounging.
So from a single Greek compound meaning 'to sit down,' English has inherited 'cathedral,' 'cathedra,' 'chair,' 'chaise,' and the geometric suffix '-hedron.' The conceptual thread linking them all is the act of sitting — on a seat, on a throne, on a base. The grandest cathedral in Christendom is, at its etymological core, a building with a chair in it.