Meaning

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

adjective not based on fact; unreal; - F.D.Roosevelt

originFrom Middle English ymaginarie, ymagynary, from Latin imāginārius (“relating to images, fancied”), from imāgō, equivalent to imagine + -ary. The mathematical sense derives from René Descartes's use (of the French imaginaire) in 1637, La Geometrie, to ridicule the notion of regarding non-real roots of polynomials as numbers. Although Descartes' usage was derogatory, the designation stuck even…

Etymology from Wiktionary, CC BY-SA 3.0