A scramble unit was a device which could be used to protect one's self from detection when accessing a computer system.
River Song neglected to use a scramble unit when she accessed Erno Balthazar's systems at Padma Observatory, as noted by Samael Avner. This resulted in her being detected and chased by people hostile to herself and Avner. (AUDIO: Last Words [+]Loading...["Last Words (audio story)"])