'Castile' originates from Spanish, specifically the word 'Castilla', where 'castillo' (from Latin 'castellum') meant 'castle'.

'Castile' changed from medieval Spanish 'Castella' / 'Castilla' (from Latin 'castellum' meaning 'castle') and passed into English usage via Old Spanish and occasional Old French forms, eventually becoming the modern English name 'Castile'.

Initially, it meant 'land of castles' (a region characterized by many castles); over time it became a proper name for the kingdom and region and later came to be used for related products (for example, Castile soap).