i (Unit Imaginary Number)
The square root of minus 1.
The symbol is "i"
When you square any Real Number you always get a positive, or zero, result. For example 2×2=4, and (-2)×(-2)=4 as well.
So how can we square a number and get a negative result? Just "imagine" that we can, and it turns out that such a number, which may seem impossible, is actually useful and can solve real problems.
See: Real Number