A uni-directional card is a card that has an intentional rotational asymmetry. Some aspect of the pattern on the back of the card was designed so that when the card is rotated 180 degrees, the pattern after the rotation doesn't match the pattern of a card that has not been rotated. It should be intuitively obvious to the casual observer that uni-directional cards have extraordinary game protection issues. My criteria for a uni-directional card is that the card must have been designed for use in live casino play. The card cannot be a novelty card. In searching for uni-directional cards, I scoured my six-book collection of distinct cards as well as my collection of over 750 distinct cards from different casinos (see this post). I also used a couple of archived cards that have been sent to me over the years.
Out of well more than a thousand distinct cards from casinos around the world, surprisingly few are uni-directional. Here is my collection of uni-directional cards (click on an image to make it larger):