A id assigned for services running on a device to allow for network communication.
Port Types
TCP or UDP
Depending on whether the service is using TCP or UDP as communication, they can occupy different port spaces. TCP port numbers are not the same as UDP port numbers. This means you can have a service on TCP port 88 and a service on UDP port 88 and they will have different communication streams.
Non-Ephemeral
Usually ports 0-1023. These ports are designated to be for specific services or protocols. i.e only HTTPS should use port 443.
Ephemeral/Temporary
Ports 1024-65535 used for temporary communication like when the client opens a communication stream to a non-ephemeral port on a server.
Security
A Port Scanner can be used to determine all active ports.