Nouns

(n)complex number, complex quantity, imaginary number, imaginary(mathematics) a number of the form a+bi where a and b are real numbers and i is the square root of -1

Adjectives

(s)fanciful, imaginary, notionalnot based on fact; existing only in the imagination“the falsehood about some fanciful secret treaties” (F.D.Roosevelt), “a small child's imaginary friends”, “to create a notional world for oneself”