WordNet sense 1 ((mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1):

complex number, complex quantity, imaginary number

WordNet sense 2 (not based on fact; dubious):

fanciful, notional, imagined

WordNet sense 3 (lacking in reality or substance or genuineness; not corresponding to acknowledged facts or criteria):

unreal