transposing instrument n (Instruments) a musical instrument, esp a horn or clarinet, pitched in a key other than C major, but whose music is written down as if its basic scale were C major. A piece of music in the key of F intended to be played on a horn pitched in F is therefore written down a fourth lower than an ordinary part in that key and has the same key signature as a part written in C Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014