A loch is the name given to a body of water in Scotland or Ireland. The spelling lough is used in Hiberno-English, although it is an anglicisation of the Irish form of the word, also "loch". The Scottish spelling is retained by many in Northern Ireland, where Ulster Scots is spoken more than Irish. The word lough is also used in Northumbria to denote such a body of water, though the pronunciation "loff" is different.

Scotland

Perhaps the most famous Scottish loch is Loch Ness, although there are other large examples such as Loch Awe, Loch Lomond and Loch Tay. For a full list, see list of Scottish lochs.

In Scotland, as well as referring to lakes, it is also used for the many fjords on the western and northern coasts (e.g. Loch Long, Loch Fyne, Loch Linnhe, Loch Eriboll). These are also known as sea lochs. These should not be confused with firths, found mainly on the east coast of Scotland, and of different geological origin.

Although there is no strict size definition, a small loch is often known as a lochan.

Some new reservoirs for hydroelectric schemes have been given names faithful to the names for natural bodies of water - for example: the Loch Sloy scheme, and Lochs Lagan and Treig (which form part of the Lochaber hydroelectric scheme near Fort William). Other expanses are simply called reservoirs, eg: Blackwater Reservoir above Kinlochleven.

Scotland has only one natural water body actually called a lake, the Lake of Menteith, an Anglicisation 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. Although, it does look more like an English lake than a Scottish loch, mostly surrounded by low-lying land rather than hills.

Ireland

Some estuaries and inlets in Ireland are also named "lough", for example Lough Swilly and Lough Foyle. Almost all, if not absolutely all lakes in Ireland are named "Lough". Lough Neagh in Ulster is the largest lake not only in Ireland but the whole of western Europe. The three on the River Shannon are Lough Allen, Lough Ree and Lough Derg. Upper and Lower Lough Erne are two consecutive lakes in Fermanagh, and area often referred to as "Ireland's lake district". For other loughs in Ireland, see List of Irish lochs and loughs.