The letter i is used to signify that a number is an imaginary number. It stand for the square root of negative one.
In electrical engineering it is often replaced by the letter j to avoid conflict with the symbol for current. See
Animated definition of complex numbers and how they can be plotted on a two-dimensional plane
An imaginary number is one that when squared, gives a negative result.
Definition and meaning of the math word j. Includes a list of other pages that refer to this word
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