Signal-to-noise The amount of useful information to be found in a given ratio Usenet newsgroup. Often used derogatorily, for example: "the signal-to-noise ratio in this newsgroup is pretty low."
SIMTEL20 The White Sands Missile Range used to maintain a giant collection of free and low-cost software of all kinds, which was "mirrored" to numerous other ftp sites on the Net. In the fall of 1993, the Air Force decided it had better things to do than maintain a free software library and shut it down. But you'll still see references to the collection, known as SIMTEL20, around the Net.
Smiley A way to describe emotion online. Look at this with your head tilted to the left :-). There are scores of these smileys, from grumpy to quizzical.
Snail mail Mail that comes through a slot in your front door or a
box mounted outside your house.
Sysadmin The system administrator; the person who runs a host
system or public-access site.
Sysop A system operator. Somebody who runs a bulletin-board
system.
TANSTAAFL There Ain't No Such Thing as a Free Lunch.
TCP/IP Transmission Control Protocol/Internet Protocol. The particular system for transferring information over a computer network that is at the heart of the Internet.
Telnet A program that lets you connect to other computers on
the Internet.
Terminal There are several methods for determining how your
emulation keystrokes and screen interact with a public-access
site's operating system. Most communications programs
offer a choice of "emulations" that let you mimic the
keyboard that would normally be attached directly to
the host-system computer.