Imaginary numbers are numbers that are made from combining a real number with the imaginary unit, called "i", where "i" is defined as formula_1. They are defined separately from the negative real numbers in that they are a square root of a negative real number (instead of a positive real number). This is not possible with real numbers, as there is no real number that will multiply by itself to get a negative number (e.g., formula_2 and formula_3). The set of imaginary numbers are sometimes denoted by the letter formula_4.