The symbol used by engineers and some physicists to denote i, the imaginary number 
. 
is probably preferred over 
because the symbol 
(or 
) is commonly used to denote current.
j
See also
i, Imaginary Number, Imaginary UnitExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "j." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/j.html