The Firth of Forth (Scottish Gaelic: Linne Foirthe) is the estuary (firth) of several Scottish rivers including the River Forth. It meets the North Sea with Fife on the north coast and Lothian on the south. It was known as Bodotria in Roman times. In the Norse sagas it was known as the Myrkvifiörd. An early Welsh name is Merin Iodeo, or the "Sea of Iudeu".
Firth is an anglicization of fjord. Forth is the name of the river; this is *Vo-rit-ia (slow running) in Proto-Celtic, yielding Foirthe in Old Gaelic and Gweryd in Welsh.