A Marilyn is a mountain or hill in the United Kingdom, Republic of Ireland or Isle of Man with a relative height of at least 150 metres (492 ft), regardless of absolute height or other merit. The name was coined as a punning contrast to the designation Munro, used of a Scottish mountain with a height of more than 3,000 feet (914.4 m), which is homophonous with (Marilyn) Monroe.
There are 2,009 Marilyns identified: 1,217 in Scotland, 453 in Ireland (of which 65 are in Northern Ireland), 176 in England, 158 in Wales, 5 on the Isle of Man. Black Mountain, in the Black Mountains, on the border between England and Wales, was formerly counted in both countries but is now treated as being in Wales only. The list of Marilyns in Britain was compiled by Alan Dawson in his book The Relative Hills of Britain, and continues to change as resurveying produces revised heights for hills and the cols between them. The list was extended into Ireland by Clem Clements in a booklet, The Hewitts and Marilyns of Ireland.
Although many of the islands' largest hills including Ben Nevis, Carrantuohill, Scafell Pike and Snowdon are Marilyns, many other large hills such as Cairngorm, a number of Munros, and other well-known hills such as Bowfell, the Langdale Pikes and Carnedd Dafydd, are not Marilyns because they do not have sufficient relative height. However, some lower hills such as Seatallan and Watch Hill on the edges of Lakeland and the Long Mynd in Shropshire do qualify because of their isolation from higher hills. Not all of the Marilyns are even hills in the usual sense: one, the highest point of the Weald, lies within the East Sussex town of Crowborough, whilst the top of the Yorkshire Wolds, Bishop Wilton Wold, lies alongside the A166 road. At the other extreme are Stac Lee and Stac an Armin, the two highest sea stacks in the British Isles, in the St Kilda archipelago, over 81 miles (130 km) west of the Scottish mainland.
Nearly 80% of Marilyns in Great Britain are in Scotland, where they logically extend earlier Scottish lists such as the Munros, Corbetts, Grahams and Donalds. All Corbetts and Grahams, 203 of the 282 Munros, and around half of the Donalds are also Marilyns.
Some hillwalkers attempt to climb as many Marilyns as possible as a form of peak bagging. Some radio amateurs attempt to operate from the summit of every Marilyn. As of the end of 2009, no one had climbed all the Marilyns in Great Britain; however, three people were only two short of completing them (because of the inaccessibility of the sea stacks on St Kilda and their protected status as part of the largest gannet nesting site in the world, maintained by the National Trust for Scotland). Two other people are only three short of completion 
- Hill lists in the British Isles
- List of mountains of the British Isles by relative height – a list of the top 119 Marilyns by relative height.
- Dawson, Alan (2006). "Update to The Relative Hills of Britain". Retrieved 2006-10-06.
- Dawson, Alan (1992). The Relative Hills of Britain. Milnthorpe, Cumbria: Cicerone Press. ISBN 1-85284-068-4.
- Clements, E.D. 'Clem' (1998). The Hewitts and Marilyns of Ireland. Cambuskenneth, Stirling: TACit Press. ISBN 0-9522680-8-6.
- Earnshaw, Jon (2006). "Summits on the Air". Retrieved 2006-01-12.
- Dawson, Alan (2009). "Marilyn Hall of Fame". Retrieved 2009-05-12.
- Marilyn News Centre
- Database of British and Irish Hills Regularly updated data on the Marilyns, HuMPs and other lists
- Lakes24 24 Marilyns in 24 hours in the English Lake District
- Winter wanderland -- Guardian travel article about the Marilyns
- Google Earth .kmz file showing all Marilyns
- List and introduction to 2,993 British "HuMPs", or hundred metre prominences.