A firth is a long, narrow arm of the sea that reaches in between two pieces of land — often the wide mouth of a river where it opens out and meets the ocean. The word belongs to Scotland. You meet it mainly in place names on a map: the Firth of Forth, the Firth of Clyde, the Moray Firth. Outside Scotland almost nobody uses it as an ordinary word, and even in Scotland people usually say it as part of a name rather than saying 'let us go and look at a firth'. So how does it differ from a bay? A bay is wide and open; a firth is long and narrow, squeezed between land on both sides, and the sea reaches quite far inland. And here is the useful connection: firth is the same word as fjord, the Norwegian word for those steep narrow sea inlets. Both came from the same old northern root, one through Norse into Scots, one straight from Norwegian. Scotland's coast has them for the same reason Norway does — ice carved the land. For a student, this is really a reading word. You need it to understand a map, a news story about a Scottish bridge, or a novel set in the north. You will almost never need to write it.