Essentially, the first printing of the first edition of a book.  Specifically, all of the copies printed from the first setting of type; can include multiple printings if all are from the same setting of type. Every printed book has a first edition, many never have later editions. A later edition would have substantial changes in the printing plates or type such as the addition of a new preface or new chapter or major changes throughout the text and often is printed from a complete resetting of the type. When book collectors use the term first edition, they are usually referring to the first printing and if there are different states or issues, the earliest of those. [R. Lucas, Essentials of Book Collecting, Part 2]

A first printing are the copies of a book or other printed material which originate from the same press run or from the same plates or setting of type at one time. In the example given for "Edition" above, the 500 copies would be the first printing and the 300 copies comprise the second printing. In the 19th century some publishers labeled later printings as if they were later editions, i.e. a second printing would be called a "second edition" on the copyright page.  [R. Lucas, Essentials of Book Collecting, Part 2]

An  issue is a portion of an edition printed or published deliberately by the printer or publisher in a distinct form differing from the rest of the printing relative to paper, binding, format, etc. The distinction between "issue" and "state" is that the former relates to changes done on purpose by the publisher or printer and intentionally treated as a separate unit, i.e. a large paper issue or an issue in publisher's leather. [R. Lucas, Essentials of Book Collecting, Part 2]

A state is the portion of a printing with changes such as minor alterations to the text either intentional or accidental; insertion of cancels, advertisements or other insertions; copies on different paper without intention of creating a separate issue; and changes other than folding or collating or binding. An example would be when a pressman discovers battered or broken type, stops the presses and resets that portion of the page by replacing the broken type and then resumes the printing, which would result in at least two states. [R. Lucas, Essentials of Book Collecting, Part 2]

A variant usually refers to differences in bindings or endpapers (paper located just inside the front and rear covers, one half of which is glued to the cover). One variant may have a title stamped on the front cover in black and another may be stamped in red.  [R. Lucas, Essentials of Book Collecting, Part 2]