a theme in a fugue that occurs simultaneously with the second and often the subsequent themes of the main subject.
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British Dictionary definitions for countersubject
music (in a fugue) the theme in one voice that accompanies the statement of the subject in another
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