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.
Some related terms:
Issue: 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 and intentionally treated as a separate unit, i.e. a large paper issue.
State: a portion of a printing with changes such as minor alterations to the text either intentional or accidental; insertion of cancels, advertisements, or insertions; copies on different paper without intention of creating a separate issue; and other changes other than folding or collating or binding.
Variants: usually refers to differences in bindings or end papers within an issue or printing. One variant may have a title stamped on the front cover in black and another may be stamped in red.