Please see also Loch

A lough is a body of water and either:

• a lake or;
• a sea lough, which may be perceived also as a fjord, estuary, bay or sea inlet.

The name is an Anglicised form of loch, which is primarily Gaelic. Loch is also used in English. Lough is applied to many bodies of water in the island of Ireland and some in Northumbria, while loch is applied to many in Scotland.

Except when individually named loughs are often referred to as lakes, fjords, estuaries and sea inlets. Thus lake district and estuary bed may be used in preference to lough district and lough bed. (This practice is not followed to anything like the same degree in English use of loch).

Almost all, if not absolutely all lakes in the island of Ireland are named as "loughs ". Lough Neagh in Ulster is the largest lake in the British Isles. 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, an area often referred to as "Ireland's lake district". For other loughs in Ireland, see List of Irish lochs and loughs.

Sea loughs include Lough Swilly and Lough Foyle.