Calling all geeks...

Howdy y’all!
I need help from one of you geek-types. What is the name for the distortion effect seen with projection systems when the screen is at an angle to the projector?
The only reason I even know there is such a name is some of the data projectors I’ve used have a little adjustment thingee on them with the name of the problem labeled on the knob.
This question occured to me in class the other day as our professor tried in vain to focus the slide projector (he was showing some histology slide so I don’t know how he tell it was out of focus to begin with)
I have no doubt that my obsession in remembering this name is a not-so-subtle way of study-avoidance but I’d appreciate the help anyway. :)
Take care,

I believe that effect is called “keystone” and most projectors come with “keystone correction”. :D