a room, as in a mental hospital, with padded walls for the confinement of violent inmates.
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British Dictionary definitions for padded cell
a room, esp one in a mental hospital, with padded surfaces in which violent inmates are placed
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012