LochA loch is the name given to a body of water in Scotland; it is also used for some large fjord-like inlets on the western and northern coasts, known as sea lochs. Although there is not distinct size definition, smaller bodies of water are often known as lochans.
Some new reservoirs for hydroelectric schemes have been given names faithful to the names for natural bodies of water - for example: the Loch Sloy pumped-storage 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.
Not all freshwater lakes are known as lochs, although those that do differ tend to have colourful histories - for example Lake of Menteith, the only lake in Scotland, has an ancient priory on an island on the lake where Mary Queen of Scots took refuge in 1547. She was only four years old at the time and stayed for three weeks after the disastrous battle of Pinkie in September of that year.
[http://www.mysteriousbritain.co.uk/majorsites/inchmahome.html Inchmahone Priory
on Lake Menteith]