Short-print: Also called an "SP," a base-set card that is printed in less quantity than other cards in the same set. Originally, short-printing was a physical necessity. For example, cards might be printed in 132 card sheets. If you have seventy-two players in the set, sixty players would appear twice on each sheet, and the last twelve players would only appear once. That results in a relative scarcity of those twelve players, which usually leads to a greater demand, and then a higher price, and ultimately to better sales in the secondary market.
Card makers have not ignored this, and in the late '90s the seeding of inserts has been carried over to cards that are ostensibly part of the base set. Locating the "SPed" cards requires greater numbers of packs and boxes being broken, and thus better sales. Think of them as a hybrid of an insert and a base set card.
Common targets at this time for SPs are rookies and stars. Historically, there was no discernible reasoning behind who got short-printed; Rip Repulski might be just as tough to find as Ernie Banks. SPed rookies tend to be the most desirable, and this can lead to great expense, especially when combined with serial numbering. Because of this, some collectors think that these cards should be treated as inserts, and thereby making them ineligible for the coveted "rookie card" status.