Imaginary number
This is why mathematicians invented the imaginary number, i, and said that it is the main square root of −1. The square root of −1 is not a real number, so this definition creates a new type of number, just like fractions create numbers like 2/3 that are not counting numbers like 4 or 10, and negative numbers create numbers that are less than 0. Sometimes, mathematicians seem rather comfortable using a number that is so unusual, but the name imaginary should not fool you because i is as valid a number as 3 or 145,379.