1. 3 meanings. First and most generally, a place where information goes into or out of a computer, or both. E.g. the serial port on a personal computer is where a modem would be connected. From Matisse

2. On the Internet, port often refers to a number that is part of a URL, appearing after a colon (:) right after the domain name. Every service on an Internet server listens on a particular port number on that server. Most services have standard port numbers, e.g. Web servers normally listen on port 80. Services can also listen on non-standard ports, in which case the port number must be specified in a URL when accessing the server. From Linux Guide @FirstLinux

3. In TCP/IP, a port is an extension of an Internet address that tells which program is to receive the data. In other words, if I send data to 192.0.2.111, port 110, then I'm talking to the POP3 e-mail service. However, if I send something to port 80 on the same machine, then I'm talking to the web server on that machine. Key point: I can have two URLs that look like http://robertgraham.com/ and http://robertgraham.com/. These two URLs access different web server programs running on the same machine, one at port 80 and that other at port 90. Misconception: Many people believe that the port correctly identifies the protocol that runs on that port. For example, the port 110 has been assigned to the POP3 e-mail service. However, even though this is the correct port for the protocol, somebody could put a different service on this port, such as HTTP. In this example, I could then supply URLs that looked like http://www.robertgraham.com:110/. From Hacking-Lexicon

4. On the Internet port often refers to a number that is part of a URL, appearing after a colon (:) right after the domain name. Every service on an Internet server listens on a particular port number on that server. Most services have standard port numbers, e.g. Web servers normally listen on port 80. Services can also listen on non-standard ports, in which case the port number must be specified in a URL when accessing the server, so you might see a URL of the form: gopher://peg.cwis.uci.edu:7000/ This shows a gopher server running on a non-standard port (the standard gopher port is 70). Finally, port also refers to translating a piece of software to bring it from one type of computer system to another, e.g. to translate a Windows program so that is will run on a Macintosh. From Matisse