a group of adjacent notes played on a keyboard instrument typically with the fist, forearm, or elbow, similar groupings also occurring in orchestral music.
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British Dictionary definitions for tone cluster
music a group of adjacent notes played simultaneously, either in an orchestral score or, on the piano, by depressing a whole set of adjacent keys
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