Hologram: A three dimensional image printed on a two dimensional surface. When Upper Deck made it's debut in 1989, it affixed a small hologram onto the back of each card to discourage counterfeiting.
Throughout the 1990s, Upper Deck, as well as other manufacturers, began to make entire cards out of holograms. The most notable of these being the 1991 Upper Deck Ted Williams "HH1" and '92 UD Hank Aaron "HH2" inserts, the 1994 SP "Holoview F/X" inserts, and the 1996 and '97 SPx sets.
But by the late 90s, the novelty of hologram cards had begun to wear off. Although Upper Deck still continues to put a small hologram on the back side of most of their cards, they haven't printed any hologram cards in years.