Typically it is a termed reserved for projection systems like LCD, DLP, and LCoS.
You see, a fixed pixel display is made up of a bunch of square shaped dots. There must be wires run to each dot to control it individually and two dots, that are side by side always have a teenie-tiny little gap between them. With LCD it is the wiring that controls the LCD. With DLP it is the individual mirrors that pivot on a 'hinge'. With LCoS it is the separation of the individual dots. If you think about it, wires are largest, little mirrors have a smaller gap, and LCoS has the smallest gap of all.
But, all these little square dots are put together to form a 1280 X 720 grid of pixels. Just like on graph paper, if you filled in all the dots, 1280 wide and 720 tall, but never drew ON the light blue lines between each block, you would see a pattern of light blue boxes that looks like a screen door. This is the screen door effect.
You really shouldn't be able to see it on a plasma. I have 20/20 vision and on my 50 inch 1365x768 display the interpixel gap (screen door) disappears about about 2 feet. On a 42 inch 853x480 display, it is about 5 feet that interpixel gap disappears. On various LCD, DLP, and LCoS rear screen products, the interpixel gap depends on the microchip used (they are different!), and the size of the screen. The further back you get, the less you will be able to see the interpixel gaps.
If you can't see the interpixel gap at any distance - the projector is out of focus. Every single pixel should be very sharp with a well defined border around it. If it is not, then light is being allowed onto the gap which is softening the image artificially.
