An MS-DOS command used to configure MS-DOS System Devices. Helpful for setting DOS to display more lines on the screen (mode con lines=50). Or redirecting print jobs to a serial printer (I suppose people actually did this years ago).

Output of the mode/? help screen:

Configures system devices.

Printer port: MODE LPTn[:] [COLS=c] [LINES=l] [RETRY=r]
Serial port: MODE COMm[:] [BAUD=b] [PARITY=p] [DATA=d] [STOP=s] [RETRY=r]
Device Status: MODE [device] [/STATUS]
Redirect printing: MODE LPTn[:]=COMm[:]
Prepare code page: MODE device CP PREPARE=((yyy[...]) [drive:][path]filename)
Select code page: MODE device CP SELECT=yyy
Refresh code page: MODE device CP REFRESH
Code page status: MODE device CP [/STATUS]
Display mode: MODE [display-adapter][,n]
Typematic rate: MODE CON[:] [RATE=r DELAY=d]

This one's for posterity