i (unit imaginary number)

The square root of minus 1

The symbol is i

It is a number that, when multiplied by itself, produces −1

But when we square any Real Number we always get a positive, or zero, result.
Examples:
2 × 2 = 4, and
(−2) × (−2) = 4 also.

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.