The first impression of the first printed edition of a book or map, unless otherwise qualified.
For example, the first commercially available edition of a book which was originally privately printed is the first trade edition; we might also use the term for the first treatment of a popular text by a fresh illustrator, and sometimes there are issue points – minor tweaks to text or binding within the same run – in which case a book might be ‘first edition, second issue’. A book which was originally published in parts, or serialised in a magazine, could have a first edition ‘in book form.’ The country and language in which a book was originally published also matters.
Serious booksellers who know their stuff will use the term ‘first edition’, however it may be expanded or qualified, to pinpoint a precise moment in a book’s publication history. Bad ones use it as a false glamour, to make themselves and their wares look more important. The difference between the two is easy to spot.
Whole books have been written on the subject of editions and states – a proper catalogue description should leave you in absolutely no doubt about what you are looking at.