Iota, also known as the symbol , is denoted as being the square root of -1. Since square roots of negative numbers do not exist, there is a variable that represents it complexly. In fact, every real number (a) can be represented using the form (a) + (0).
On occasion, variables can also use and it does not have to be talking about the square root of -1. However, this rarely ever happens to avoid confusion. Check instructions in questions to see how is defined.