(1) The number that identifies Internet packets as Web transactions (HTTP transactions).  All port numbers reside in the header of the packets, and requests that are marked port 80 are processed by the Web server (see 
TCP/IP port).  Port 80 requests may activate any number of different server-side processes before a Web page or some other file is returned to the user.
Port 443 Is the Secure Version
Port 80 represents the non-secure HTTP protocol, while port 443 is HTTPS, the secure version.  Today, all Web sites should be configured for HTTPS, otherwise they are vulnerable to hacking.  For a list of common port numbers, see 
well-known port.  See 
port forwarding, 
HTTP and 
HTTPS.
(2) In a PC, port 80h is an input/output address that is used to send power-on codes.  See 
POST code and 
PC I/O addressing.