*Integration of Alternative Sources of Energy*and I'm having trouble with the big words.

The presented equations often use "j" in conjunction with reactance. Examples: jXr0 where Xr0 is the reactance of the blocked rotor; jX2 where X2 is the stator reactance; jXr where Xr is rotor reactance; jXm where Xm is equivalent reactance of iron in the stator.

Is "j" a calculus notation I might be unaware of? Is it some universal thing I missed somehow? I've read the chapter three times now, and I can't find "j" except in front of the occasional "X."