Imaginary numbers are numbers than when squared equal a negative number, as in i^2 = -1, where i is the imaginary number. You'll also often see them represented as i = √-1 (that's the square root of -1).\nDon't be confused by the poorly chosen name - imaginary numbers do indeed exist and are used in advanced math, such as in the physics of electromagnetic fields. The analogy that Wikipedia uses is a good one - just like you don't need the concept of fractions to count stones, it doesn't mean that fractions don't exist. :)
What is an "imaginary number"?