Greek mathematician Heron of Alexandria is credited with having first conceived of imaginary numbers about the time of Christ, but it was Rafael Bombelli who first described the rules for the use of complex numbers in the late 16th century. At this early period of history, complex numbers were not well understood, with many mathematicians viewing them as useless.
The square root of a nonpositive real number is an "imaginary number." While an imaginary number can be added to a real number to create a complex number, such numbers were considered of no value, and early mathematicians were slow to adopt their use. In fact, the term "imaginary number" was meant to be derogatory. In modern times, however, imaginary numbers are an essential component of scientific and other calculations.