| Noun | 1. | pure imaginary number - an imaginary number of the form a+bi where a is 0 complex number, complex quantity, imaginary, imaginary number - (mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1 imaginary part, imaginary part of a complex number - the part of a complex number that has the square root of -1 as a factor |