I represents the imaginary unit .
Numbers containing I are converted to the type Complex.
I can be entered in StandardForm and InputForm as , ii or \[ImaginaryI].
, jj and \[ImaginaryJ] can also be used.
In StandardForm and TraditionalForm, I is output as .
See The Mathematica Book: Section 1.1.3, Section 1.1.5 and Section 3.2.8.
See also: Re, Im, ComplexExpand, GaussianIntegers.