A term used to describe used books (probably new books too, but the only way to get new book club editions is through a book club, in which case you don't need to be told...) These books are generally cheaper, later editions, sold through mail order and available only to members of the book club (which is to say, those who signed up for the book club's mailing list).
Book club editions tend to be of poorer quality, slightly smaller, and of almost no interest to the majority of collectors. If you're selling books on-line, you should always specify if your hardbacks are bookclub or trade editions. (With trade paperbacks it rarely matters, and with mass-market paperbacks it never matters).
To find out if you have a bookclub edition, look at the dust jacket. if it has a price, it's trade, if it doesn't, it may very well be bookclub (It might also be price clipped, or maybe just unusual).
Next, look at the size. Bookclub hardback fiction
books tend to be around 13 1/2 X 20 3/4 CM
, or 5 1/4 X 8 1/4 Inch. Trade are most often about 15 1/2 X 23 1/2 CM, or 6 1/4 X 9 3/8 Inches.
Usually you will have lots of other bookclub editions to compare yours to, and it will be easy to determine a fair price (probably a couple of dollars). If you have something unusual, it may be that the bookclub edition is actually somewhat valuable, although still less so than most editions.