A Marilyn is a hill of any height with a drop of 150 metres or more on all sides. In other words, a relatively high hill. The British Marilyns are all listed in a book called The Relative Hills of Britain (RHB) by Alan Dawson, published by Cicerone Press in April 1992.
The total number of Marilyns is currently 1556. Recent changes are the promotion of Creag an Amalaidh (16D), Creag na h-Eararuidh (1B), Creag Toll a'Choin (12A), Kirriereoch Hill (27B), and Sithean Mor (13A) to Marilyns, and the demotion of Beinn Dearg (1B), Maoile Lunndaidh (12A), Stob na Boine Druim-fhinn (19C), Swyre Head (41) and Troweir Hill (27B). More details are available in the latest update to RHB.
On 13 October 2014, Rob Woodall and Eddie Dealtry climbed Stac an Armin and then Stac Lee, and so became the first people to climb all the Marilyns. On 17 October 2015 twelve people reached the summit of Stac Lee and the following day eleven of them climbed Stac an Armin, with Michael Earnshaw and Alan Whatley becoming the third and fourth people to climb all the Marilyns.