Im { x } is the imaginary component of complex number x.
For example:
If s = a j + b, where j = sqrt(-1), then:
Im { s } = a
The imaginary component of any real number n is zero, the imaginary comonent of any imaginary number s is s / j.
Compare to Re, the real component. ((Im { s })(j) + Re { s } = s.)
(This is sometimes typeset using a calligraphic capital I.) | 
