imaginary unitn. Symbol i The square root of -1, corresponding to the point (0,1) in the geometric representation of complex numbers as points in a plane.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.imag?inary u?nit n. the complex number i. [1905?10] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.