Loch Lomond (ˈlɒx_ˈloʊmənd; Loch Laomainn - 'Lake of the Elms') is a freshwater Scottish loch which crosses the Highland Boundary Fault, often considered the boundary between the lowlands of Central Scotland and the Highlands. Traditionally forming part of the boundary between the counties of Stirlingshire and Dunbartonshire, Loch Lomond is split between the council areas of Stirling, Argyll and Bute and West Dunbartonshire. Its southern shores are about northwest of the centre of Glasgow, Scotland's largest city. The Loch forms part of the Loch Lomond and The Trossachs National Park which was established in 2002.
Loch Lomond is long and between wide, with a surface area of . It is the largest lake in Great Britain by surface area; in the United Kingdom, it is surpassed only by Lough Neagh and Lough Erne in Northern Ireland. In the British Isles as a whole there are several larger loughs in the Republic of Ireland. The loch has a maximum depth of about in the deeper northern portion, although the southern part of the loch rarely exceeds in depth. The total volume of Loch Lomond is , making it the second largest lake in Great Britain, after Loch Ness, by water volume.
The loch contains many islands, including Inchmurrin, the largest fresh-water island in the British Isles. Loch Lomond is a popular leisure destination and is featured in the song "The Bonnie Banks o' Loch Lomond". The loch is surrounded by hills, including Ben Lomond on the eastern shore, which is in height and the most southerly of the Scottish Munro peaks. A 2005 poll of Radio Times readers voted Loch Lomond as the sixth greatest natural wonder in Britain.
The depression in which Loch Lomond lies was carved out by glaciers during the final stages of the last ice age, during a return to glacial conditions known as the Loch Lomond Readvance between 20,000 and 10,000 years ago. The loch lies on the Highland Boundary Fault, and the difference between the Highland and Lowland geology is reflected in the shape and character of the loch: in the north the glaciers dug a deep channel in the Highland schist, removing up to 600 m of bedrock to create a narrow, fjord-like finger lake.