Origins
The word 'church' is one of the earliest Greek loanwords in the Germanic languages, testifying to the contact between Germanic peoples and Greek-speaking Christianity during the late Roman period. It descends from Old English 'cirice' or 'cyrce,' from Proto-Germanic *kirikō, borrowed from Greek 'kyriakón' (κυριακόν), a neuter adjective meaning 'of the Lord,' used as a shortening of 'kyriakón dōma' (the Lord's house). The underlying noun 'kýrios' (κύριος) meant 'lord, master, one with authority,' and in Christian usage became the standard Greek title for God and Christ.
The borrowing of 'kyriakón' into Proto-Germanic must have occurred relatively early — probably in the third or fourth century CE — because the word is attested across all the Germanic branches: Old English 'cirice,' Old High German 'kirihha' (modern German 'Kirche'), Old Norse 'kirkja,' Old Saxon 'kirika,' Old Frisian 'zerke' and 'tsiurke,' Dutch 'kerk,' and Scots 'kirk.' The phonetic development from 'kyriakón' to *kirikō involves the simplification and adaptation of the Greek word to Germanic phonological patterns.
A striking feature of European church vocabulary is the split between Germanic and Romance languages. Germanic languages borrowed 'kyriakón' (the Lord's house), focusing on the building. Romance languages borrowed a completely different Greek word: 'ekklēsía' (ἐκκλησία, assembly, congregation), from 'ek-' (out) + 'kalein' (to call) — literally, 'the called-out ones.' Latin 'ecclēsia' became French 'église,' Spanish 'iglesia,' Italian 'chiesa,' Portuguese 'igreja,' and Romanian 'biserică' (this last from Latin 'basilica'). This lexical split reveals a fundamental difference in what the two language groups emphasized when they adopted Christianity: the Germanic peoples named the faith after its building; the Romance peoples named it after its people.
Germanic Development
The Scots word 'kirk,' which preserves the Proto-Germanic 'k' sound more faithfully than southern English 'church' (where the 'k' was palatalized to 'ch'), gives its name to numerous Scottish place names and surnames: Kirkwall, Kirkpatrick, Kirkcaldy, Dunkirk (Flemish 'Duinkerke,' 'dune church'). Captain Kirk of Star Trek was reportedly named for this word by Gene Roddenberry.
The Greek 'kýrios' (lord) also survives in the liturgical phrase 'Kyrie eleison' (Κύριε ἐλέησον, 'Lord, have mercy'), one of the few Greek phrases preserved in the Latin Mass and still used in Christian worship worldwide. The phrase connects 'church' etymologically to one of the oldest prayers in Christian tradition — both derive from the same Greek root naming the lord to whom the house and the prayer are addressed.
The PIE root *ḱewH- (to swell, to grow strong, to have power), which underlies Greek 'kýrios,' also connects to concepts of physical and social growth. To be a 'kýrios' was to be a person of swollen importance — one who had grown powerful enough to exercise authority. The church, etymologically, is the house of the one who swelled to supremacy.