Technically, in bookselling terms (historically), "First Edition" (without qualification) means 1st ed./1st impression (or "printing" if you want to use US terminology). The term "thus" is also used (e.g. "First Edition thus" to identify a 1st/1st where the seller is aware (or perhaps unsure) that there were earlier editions i.e it's a first impression of a particular incarnation. (Bibliographically it's a different matter.)

However (as I alluded to in another post) Abebooks is riddled with sellers who don't know or don't care about using correct terminology --& as a result you're forced to get everything clarified, less you end up with a later impression book, when the seller clearly stated "First Edition". Good sellers clearly clarify for you e.g. "First Edition (& First Impression)".

BH