Imaginary Numbers
Imaginary numbers exist.
The name is unfortunate. When they were proposed, someone used the term "imaginary" to make fun of it, and the name stuck. They could have just as easily been called something less whimsical, like "auxiliary" or "orthogonal" numbers. But they are as real as negative numbers or fractions are.
Negative numbers let you solve , whereas imaginary numbers let you solve .
Imaginary numbers and real numbers are the two starting sets that when combined make up complex numbers (ie. complex numbers are the union set of all real numbers and all imaginary numbers.