A term with a number of meanings:

1. TCP process identification number

TCP/IP has ports which when associated with an IP identify uniquely one end of a TCP (or UDP) connection. Some ports are 'well known' and by convention programs listen on them, for instance port 22 by convention will have an ssh daemon listening on it. services(5) lists all these ports.

The best analogy I've seen for this is where a business has one telephone number (which corresponds to your computer's IP address), and then every service/person in the business has their own extension number (which corresponds to a service on the computer using a specific port number).

2. I/O address

The range of I/O ports is a separate address space to talk to I/O devices. AddToMe

3. Adjustment of software for different environments

Porting is making software that runs on one type of computer or OperatingSystem run on another. Programs designed to make this process easy are said to be portable. The success of the Unix OperatingSystem is largely due it being very portable.

4. BSD's package management system

As ports(7) says:

The FreeBSD Ports Collection offers a simple way for users and administrators to install applications. Each port contains any patches necessary to make the original application source code compile and run on BSD. Compiling an application is as simple as typing make build in the port directory! The MakeFile automatically fetches the application SourceCode, either from a local disk or via FTP, unpacks it on your system, applies the patches, and compiles it. If all goes well, simply type make install to install the application.