Is it ensure, insure, or assure?
early 15c., in Scottish, from Middle French domaine "domain, estate," from Old French demaine "lord's estate," from Latin dominium "property, dominion," from dominus "lord, master, owner," from domus "house" (see domestic). Form influenced in Old French by Medieval Latin domanium "domain, estate." Internet domain name attested by 1985.
domain do·main (dō-mān')
One of the homologous regions that make up an immunoglobulin's heavy and light chains and serve specific immunological functions.