A Short Print, also sometimes notated as an SP is a card that has fewer copies than other cards in the same set. Essentially a shorter print run was once caused by cards being printed on sheets with a number of cards per sheet that did not evenly divide into the total count of the set. Thus fewer of each card was produced, making them more scarce.
For example: the 1948 Bowman baseball card set was printed on 42 card sheets, however the card set had 48 cards. Therefore 12 cards were short printed (6 cards were exchanged for 6 different cards on the sheet halfway through the printing), the other 36 cards in the set were printed approximately twice as often.
In modern card sets, Short Prints are included on purpose rather than as a simple effect of the printing process. For example: in the 2001 Bowman Heritage Baseball set, the last 110 are short printed and are inserted at a rate of approximately 12 per box, a much lower rate than the other 330 cards.