i (Unit Imaginary Number)

The square root of minus 1.

Now, if 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? Because we "imagine" that we can … and it turns out that such a number, which may seem impossible, is actually useful and can solve real problems.

i is the "unit" imaginary numbers (the same as "1" for Real Numbers) and equals √(-1) (the square root of minus one)

See: Imaginary Numbers