I'm sure that there must be a better term for this. By the 'Key Episode' of a (hit) TV series, I mean not a dramatic pivot. Rather, the episode, early on in the series when the drama leaps from a basic expectation-fulfilling quality into a show that is brilliant, original, exhilarating and destined to stay like that for successive re-commissioning.
Oft-cited is the example of episode 5 of the first series of The Sopranos, College, in this respect. This is the episode in which Tony Soprano is first seen to kill a man (with his bare hands, and whilst he's trawling round prospective colleges with his daughter). This 'Key Episode' shows that The Sopranos means business, i.e. it's going to be hard-hitting but successful in twinning that with the strangely warped version of suburban, kitchen-sink drama that is the mainstay of comparable but inferior dramas.
Tonight I saw the 6th episode of Nurse Jackie, Tiny Bubbles. This must surely be the Key Episode of this black nursing comedy, hitting the critical mass of free-flowing humour that opens up the utmost seriousness in the belly of its situation (an American ER). It strikes me as not at all surprising that Edie Falco, the eponymous star of this show, was also a significant cast member of The Sopranos.
I'm trying to think of what the key episode of other shows I have seen in their entirety might be (i.e. The Wire and The West Wing). Maybe I'll open up these specific questions - and indeed the broader one of whether there is a better/alternative/standardised term for 'Key Episode' - on Twitter.