An terminal emulation standard pushed forward by IBM throughout the years, starting with the System/38 and is still available on the AS/400 and S/390. It's related to the other 3270 standard with the exception that 5250 also allows better communication the the hardware of the terminal, not just the display, and it has better operability with the EBCDIC stuff Big Blue is still so fond of. Yes, it's "better" than 3270, but it's nowhere near as popular so it's hard to find emulators for it apart from IBM's own Client Access. Commonly found in old shops that still have miles of Twinax wiring.