Short for instant message its kinda like E-mail but faster. It is also comparable to a private message or a wisper.

Also, an (once commonly used) abbreviation for InterMail, a (once commonly used) front-end mailer for bbs systems.

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.)