The double-struck capital letter I, 
,
is a symbol sometimes used instead of 
for the ring of integers.
In contrast, the lower case symbol i is used to refer
to the imaginary unit 
.
I
See also
i, ZExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "I." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/I.html