An Etymological Dictionary of the German Language/Dom
Dom, m., ‘cathedral, dome, cupola,’ ModHG. only, borrowed from Lat. domus (for domus dei; comp. the Goth. word gudhûs, ‘the house of God, church’). An earlier loan-word is OHG. tuom (also dôm), MidHG. tuom, ‘a bishop's collegiate church, cathedral,’ which was naturalised in Germany about the 9th cent.; comp. OHG. scuola from Lat. scŏla, as if it were scôla; so tuom for tôm from dŏmus; see Schule. The form Tum, developed from MidHG. tuom, kept its ground till the beginning of the last century.