There are several terms used in the paperback book trade to indicate various types of editions.
Paperback original (PBO) – this is a book that did not originally appear as a hardcover edition.
First paperback edition or first paperback printing – books issued after the hardcover version (or in pulps and digests).
First edition thus – a book that may be (1) a collection of short stories that never before have appeared together in book form; (2) a retitled book by its original publisher or a new company; or, (3) a reissue of the original book text with new cover art.
First printing – the number of books os a title originally in print. If it sold out, the publisher would follow up with a subsequent number of books, called a second printing, etc. (All things equal, the value of a subsequent printing is much less than a first.)
A paperback original (also called PBO, original, or first edition) may be determined by looking on the back of the title page for any previous publishing history. In some cases, the cover may say ‘original’ or ‘first book publication.’ Also, check the back of the title page for 1st printing information. The best indication of this is when prior publishing history refers only to a hardback edition. Each publisher has their own way of providing a publishing history, so it is sometimes difficult to determine the printing. For example: Dell and Pocket Books are very good with this information; Gold Medal often gives only the original copyright date.
Courtesy of Huxford’s Paperback Value Guide