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Updated 06/18/2013

Firth is the Scots word used to denote various coastal waters in Scotland. It is usually a large sea bay, which may be part of an estuary, or just an inlet, or even a strait. It is cognate to fjord (both from Proto-Germanic *ferþuz) which has a more narrow sense in English, whereas 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 Lorne 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.

A firth is generally the result of ice age glaciations and is very often associated with a large river, where erosion caused by the tidal effects of incoming sea water passing upriver has widened the riverbed to an estuary. Demarcation can be rather vague. The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow, while locally the change is held to be at the Tail of the Bank where the river crosses a sandbar off Greenock at the junction to the Gare Loch, or even further west at Gourock point.

However, some firths are exceptions. The Cromarty Firth on the east coast of Scotland, for example, resembles a large loch with only a relatively small outlet to the sea and the Solway Firth and the Moray Firth are more like extremely large bays. The Pentland Firth is a strait rather than a bay or an inlet.

So what’s a loch?  A loch is either a lake or a sea inlet, which may be also a firth, fjord, estuary or bay.  Scotland has only one natural water body actually called a lake, the Lake of Menteith, an Anglicization of the Scots Laich o Menteith meaning a "low-lying bit of land in Menteith", and applied to the loch there because of the similarity of the sounds of the words laich and lake. The Lake of the Hirsel, Pressmennan Lake and Lake Louise, (In the grounds of Skibo Castle), are other bodies of water in Scotland which are called lakes and all are man-made. Most Scots will be quick to correct anyone who refers to "lochs" as "lakes".  As a name element Loch has become Lough for many bodies of water in Ireland and for some in the north of England.