This name for a body of water is Gaelic in origin and is applied to most lakes in Scotland and to many sea inlets in the west and north of Scotland. For a list, see List of lochs in Scotland.
As a name element Loch has become Lough for many bodies of water in Ireland and for some in the north of England. However, reference to these as lochs or loughs (lower case initial), rather than as lakes, inlets and so on, is unusual. For lists, see List of Irish loughs and List of English loughs.
Perhaps the most famous Scottish loch is Loch Ness, although there are other large examples such as Loch Awe, Loch Lomond and Loch Tay.
Examples of sea lochs in Scotland include Loch Long, Loch Fyne, Loch Linnhe, Loch Eriboll.
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 Laggan 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. The Lake of the Hirsel, Pressmennan Lake and Lake Louise, (In the grounds of Skibo Castle), are the only 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".
The word "loch" is used as a shibboleth to identify natives of England, because the hard "ch" sound is used in Scotland whereas most English people pronounce the word like "lock".