If it's a mainframe, then the campus is the
enclosure,
while if it's a desktop, it's pretty obvious what that is.
Today that campus worth of hardware is emulated in the hercules
s360/370/390/ zSeries emulator
http://www.conmicro.cx/hercules
running on a PC running linux or winbloZ
OS, DOS, MVS, VM ... running on your pc, serves x3270
terms over the network etc etc.