The exercise of dimming your macbook's backlight fully so that you can see your reflection on the now black screen.
Mac popularity has made this an increasing trend. Particularly amongst University girls who can often be seen attempting to dim their screens without drawing attention to themselves, and then pressing their faces up to the screen, re-applying makeup, fixing their hair, or simply checking themselves out.
Stu: hey john, check the girl next to you...totally using her macmirror
John: haha, not like a few minor adjustments in front of the macmirror are gonna fix her melted face anyways