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. Details of subsequent changes have been published in a series of update sheets.
The total number of Marilyns is now 1556, following the demotion of Cruim Leacainn (4A) and Troweir Hill (27B) and the promotion of Creag an Amalaidh (16D). Details up to April 2014 are available in the latest update to RHB.