An unwanted spot on a page printed by offset, resulting from a bit of dust or dirt on the plate. If the dirt stays there it attracts an ink build up, sometimes printing as an open circular dot, hence its alternative name of bull’s eye or fish eye. I’ve seen one which looked like (and obviously once was) an insect.
In this photo you can see a hickey in the page on the left, next to Sergey Semishkur’s name. On the right hand page, from another program, the pressman has noticed the hickey, and cleaned the plate (or of course the one on the right printed first, before the hickey arrived). Clicking on the photo will make it larger.
Making Book 
[…] plate, or even a fly, and gets squashed onto the blanket where it starts printing. This we call a hickey — and as soon as they notice it the press minders will start pulling pages out of the press and […]