the coefficient b in the complex number a + bi.
Compare real part.
Origin of imaginary part
First recorded in 1925–30
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British Dictionary definitions for imaginary part
the coefficient b in a complex number a + i b, where i = √–1
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