Two characters, usually two characters sharing a large amount of animosity, get locked in a room (safe, elevator, etc.) and must spend the episode alone together. During which time they learn a new appreciation for each other.Wikipedia has Bottle Episode.
Most bottle episodes are shot on sets already built for other episodes, frequently the main interior sets for a series, and they consist largely of dialogue or scenes for which no special preparations are needed... "A Night In", the third episode of prison sitcom Porridge...The fourth-series Doctor Who episode "Midnight" on a bus ... Star Trek "Balance of Terror".Speaking personally, these are often my favourite episodes. Breaking Bad Episode 2-10 "4 Days Out" (cf 'A Night In'?) takes place in a bus stranded in the desert, surely a homage to bottle episodes we have loved. It is one of my favourite episodes to date.
Episode 3-10, 'Fly' is also a locked room/ bottle episode, even more stark and delimited. It features the two lead actors locked in a laboratory, trying to kill a fly. That's all that happens. It could be a one-act play. After two fairly pedestrian episodes, which I didn't bother to write about, this is bold and spartan.
I have often thought that Breaking Bad is rather like Cronenberg's The Fly ('Be afraid, be very afraid'). Jeff Goldblum is definitely improved by an admixture of flyness. At first. He's randier, stronger, has more enthusiasm for life. And at some point along the process his trajectory reaches an apex, and begins to plummet. I think we start to get an inkling that things aren't going so well when he snaps that guy's arm off.
It is not a coincidence that this episode is called 'Fly'. Walter White has a long speech, the apex of the episode, where he asks himself 'Where was the balance point', where was the perfect spot, when the improvement to his fortunes was at its maximum and the degeneration had not yet become irrevocable. On that day, he says, I should have died.