#retcanon

The Star Trek universe has been experiencing timeline alterations since the very first time-travel episode in TOS Season 1.

The Doylist explanation, of course, is that they were still working out the vague details of their Generic Space Opera Setting, but from a Watsonian perspective, the now-familiar elements of what we now think of as "the Star Trek Universe" don't really start settling in until after "Tomorrow is Yesterday" (Season 1, Episode 19). Immediately after, in fact: I think that the very next episode, "Court Martial" (S1 E20), is the first time we hear the term "Starfleet" used to describe the uniformed service under which Kirk's Enterprise operated. Previous episodes bandied about terms like "Star Service", "Spacefleet Command", "Space Central", and everyone's favorite, the "United Earth Space Probe Agency". The first mention of the "Prime Directive" comes in "The Return of the Archons" (S1 E21), and I don't think we hear about the "Federation" until "A Taste of Armageddon" (S1 E23).

Even with all these key pieces on the table, though, there are still some peculiarities. Starfleet is sometimes referred to with the definite article, and isn't quite a compound noun yet: "The Star Fleet". There are a few other odd quirks that I noted at the time but don't quite recall right now, but ultimately, it's not until after "The City on the Edge of Forever" (S1 E28) -- another time travel episode, and one that definitely left some butterfly effects on the timeline -- that things start to feel like the Star Trek setting that we all know and love -- making it a milestone in more ways than one.

Doylisticly, of course, a big part of that is that there was only one more S1 episode after "City", and then they had a production break of several months to finally pull all of these ad hoc script elements into a coherent, largely consistent setting.