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 1557. Details are available in the latest update to RHB, November 2013