Cardiff - Capital of Wales Information & Resources

"The origin of the name of the city of Cardiff is subject to much ambiguity. Cardiff is the Anglicised version of the Welsh name “Caerdydd”. “Caerdydd” is split into two words; “Caer”meaning ‘fort, and “Dydd” or “Diff”, which is thought by some to refer to the river Taff on which the castle of Cardiff stands. Others, however, take it to refer to the Roman general Didius, who was governor of nearby provinces."

http://www.cardiffworld.com/