I was browsing the Wikipedia when I ran across this term, The Dormition of Theotokos, in the article on the Assumption of (the Virgin) Mary (a dogma of the Roman Catholic Church and a popular subject for religious paintings at one time). Where to start such a lovely lot of words: (1) Greek Theotokos. literally 'the childbirth, parturation of god', and an Orthodox synonym for Mary; (2) dormition, literally 'a falling asleep' here a euphemism for 'death' (a Latin calque of Greek koimisis, related with English cemetary); (3) assumption 'reception'; bonus words, Great Paraklesis and Small Paraklesis, from Greek παρακλησις (paraklēsis) 'summons; invocation; demand; exhortation; supplication'.
Here we go again on the definition of what is a word. It's a Greek word that has been used in English, so you could argue that it is an English word. Alternatively you could argue that it's not. You pays your money and you takes your choice...
Build a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life.
I'm sure there are Greek Orthodox folks who use it in conversation in English. Two hints that we're dealing with English here is that Greek speakers wouldn't say dormition, and wouldn't use the English preposition of (they would probably use the genitive case: κοιμεσις θεοτοκου (koimesis theotokou)). That's Classical Greek; not sure what it'd be in either of the two modern Greeks. (I wonder if Metamuse'll stop by some day?)