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. Topps Allen and Ginter Baseball and Topps Heritage Baseball include these types of short prints every release.
Short Print Variations are another modern card set homage to older sets. In vintage sets, occasionally there would be a variation, not considered an error, when sets went through second and third printing batches - whether it was a particular color in a border or font color. Modern sets use gimmicks like multiple photos, effects that have been inserted using Photoshop, or intentional errors. This leads to cards like 2019 Topps Heritage Base 485, where each variation is a short print in lower quantities than the regular card.