tn3270 is a
terminal emulator for communicating with
IBM mainframe computers over
TCP/IP. Like
telnet, it provides an interactive session on another computer, and in fact, it uses the telnet protocol to control the connection. It acts like an old-fashioned,
green screen 3270 series terminal; the program maps
SNA concepts like Logical Units to TCP/IP IP addresses, and translates the screen based input and output to the UNIX terminal commands to simulate a 3270 session.
The first tn3270 was part of BSD Unix. Many commerical and non-commerical tn3270 programs based on the BSD version now exist, with various extra features and extenstions. A graphical version called x3270 also exists; it is better able to handle parts of the 3270 character set that don't map to ASCII.