A computer terminal that uses the serial ports as the physical link for input.

As old as (http://wikipedia.lotsofinformation.com/wikipedia/index.php?title=Methuselah Methuselah) but still useful for accessing remote computers (and that includes some access points - ([minitar]).

It's used when there is no need for a physical terminal (monitor and keyboard) or it would be impractical to have one and while it's a slow protocol, when compared to Ethernet, it's fast enough for typing and text display.
While [Telnet] or [SSH] are well suited to the task they require a working ethernet connection, a serial links offer redundancy as it uses the serial port (ttyS- or Com port) . For  a computer/access point located in an inaccessable location (mounted on a mast or in a roof cavity) redundancy becomes especially important :-)

Under [Linux], the kernel may be compiled to load a (http://www.tldp.org/HOWTO/Remote-Serial-Console-HOWTO/ serial as console) on boot up. This allows the bootloader (http://www.znark.com/tech/serialconsole.html Grub) or Lilo to be accessed and the display of all the boot messages via the remote serial terminal.