Here are the definitions of some common pre-war card acronyms and collecting terms (and collecting terms in general).
American Card Catalog (ACC) – The book written by Jefferson Burdick that provided classifications for various sets (i.e. T206, E90, etc.). Here is a tutorial on understanding the most important book in the hobby.
Beater – A heavily worn card; considered by most to be a very low-grade poor card. May be missing part of the card, heavily scribbled upon, heavily creased, glued to something else, etc.
BGS – A third party grading company that grades the condition of cards for a fee
Cabinet Card – Photographs that were glued to some sort of cardboard backing. Families used these to display pictures of relatives but many cabinet cards were also made of baseball players. Collectors differ as to if these should be considered true baseball cards.
Colored – Colored cards refer to those that have had areas colored in to improve their appearance
Die-Cut – A card that was intentionally cut around the specific shape or outline of the player on the card
Encapsulated – A card that has been placed in a case and graded by a third party card grader (PSA, SGC, BGS, etc.)
Graded – Most commonly, this refers to a card that has been reviewed by a third party grading company (PSA, SGC, BGS, etc.) and assigned a numerical graded based on its condition
Jefferson Burdick – An early collector that was the author of the American Card Catalog
Overprint – Printing or stamping on a card for a deliberate purpose – often refers to cards that have been stamped by advertisers to add their company name to it for their own distribution. On T206 cards, it refers to cards that have had the original factory number covered and a new one added.
Pre-War – This definition varies by collectors. Some identify it as the era of cards prior to the start of World War I or World War II. Some identify it as the era prior to the end of either of those. Most definitions consider it to be either pre-1940 (the start of World War II) or pre-1945 (the end of World War II).
Premium – Premiums were items distributed by certain companies, often as part of redemption programs. Often, they were items such as photographs or albums.
PSA – A third party grading company that grades the condition of cards for a fee
Rebacked – A card that has had the original back removed and a new one added
Regionals – Regional cards are those that were printed in a specific area. Many times, sets were produced by companies to feature local teams and were only distributed within the confines of that city or geographic area.
RPPC – Stands for ‘Real Photo Post Card’ – postcards that utilized real pictures
Scraps – Sometimes called ‘Printer’s Scraps,’ these are cards that were incorrectly printed or cut and likely not intended for distribution. While they are often lacking in appearance, they can be quite valuable because they are anomalies.
SGC – A third party grading company that grades the condition of cards for a fee
SL – Used for T206 cards to indicate a Southern League player
Slabbed – A card that has been placed in a case and graded by a third party card grader (PSA, SGC, BGS, etc.)
Short Print – Cards that are a bit harder to find because the manufacturer didn’t make as many of them as they did other cards in the set
Soaking – The process of submerging a card to remove paper or residue from the back – this is a common practice to remove cards from scrapbooks or other paper where they have been glued upon
Strip card – Cards that were printed one after another in long, horizontal or vertical strips. These were hand cut by retailers, who would sell them or give them to consumers for purchases of goods.
TPG – Stands for third party grading company – a company such as PSA, SGC, or BGS that will grade the condition of a card for a fee
Trade Cards – Cards distributed by advertisers to promote a business. Often, these included pictures of sports scenes on one side and the company’s name printed somewhere on the front or back.
Trimmed – A card that has been partially cut around the edges to make the corners appear sharper
Type Card/Collecting – Type collectors acquire as many different types of different cards as possible. They generally are not concerned about completing entire sets – they merely want one card from different sets.