The term 'ionising radiation unit' is used to refer to a unit of measurement for the amount of ionising radiation present in a given area. The three crossword solutions for this term are REM (3 letters), Rontgen (7 letters) and Roentgen (8 letters). REM stands for 'Roentgen Equivalent Man', which is a unit of radiation dose equivalent to the absorption of one roentgen of gamma or X-rays by a human body. Rontgen and Roentgen are both named after Wilhelm Röntgen, who discovered X-rays in 1895.