noun - Firth is the word in the Lowland Scots language and in English used to denote various coastal waters in Scotland and England. In mainland Scotland it is used to describe a large sea bay, or even a strait.
In the Northern Isles it more usually refers to a smaller inlet. It is linguistically cognate to Fjord which has a more constrained sense in English; a firth would most likely be called a fjord if it were situated in Scandinavia. Bodies of water named "firths" tend to be more common on the east coast, or in the southwest of the country, although the Firth of Lorn is an exception to this. The Highland coast contains numerous estuaries, straits and inlets of a similar kind, not called "firth", e.g. the Minch, and Loch Torridon; these are often called sea lochs.