The Modern Library is almost entirely a collection of reprints. So a Modern Library First Edition is the first published appearance of a title in the Modern Library series.

Perhaps the only Modern Library true first in the strictest sense of the term is Styron's The Long March, ML Paperback P22. Of course, many titles had introductions especially written by the authors for the Modern Library. Some would argue that the illustrated Don Quixote is also a true first because it represents the first appearance of that title with Dali illustrations, while others would call it a First Thus because the Cervantes text remains unchanged with just the llustrations added. But these few exceptions don't change the main point that the Modern Library is by and large a reprint series.

The fact that Modern Library is a reprint collection accounts in large part for why collecting the series is such an accessible hobby. For less than $100 you can get a Modern Library first of Pynchon's V in NF/NF condition; a true first of the same title (J.B. Lippincott, 1963, 8vo., 492 pp. fully bound in violet, etc.) in not nearly as good condition will likely cost you ten times as much.

Having said that, collecting Modern Library "First Thus" titles is still a great hobby -- if what you're getting is The Real Thing. Verifying a Modern Library first edition can be tricky.

In an ideal world, all first editions would identify themselves as such on the copyright page. You'd always find "First Modern Library Edition" or "First Printing" or something comparable. But not all Modern Library first editions carry such first edition slugs, and not all Modern Library books that have such slugs, most notoriously Bemelmans' My War with the United States and Thurber's Thurber Carnival as well as post-1963 editions, are necessarily first editions.

You can use the catalogs on the inverse of dust jackets and at the book's front (early Boni-Liveright titles) or back (later titles through 1963 and sometimes later) to tell the date a piece was printed. In tougher cases you can verify that the binding type of the book matches a date range.

At any rate, the first place to look for first edition points is Henry Toledano's Modern Library Price Guide. Be sure to check out the First Edition FAQ questions at ModernLib, the Modern Library Collecting web site.