n. A number that must be expressed as the product of a real number and i, where i 2 = –1. The sum of an imaginary number and a real number is a complex number. Although imaginary numbers are not directly encountered in the universe (as in “1.544 i megabits per second”), some pairs of quantities, especially in electrical engineering, behave mathematically like the real and imaginary parts of complex numbers. Compare complex number, real number.