At the electrical level, a port consists of an interface and a protocol.
The interface is the set of pins or wires that connect the components.
The protocol defines the rules for changing the logic levels and the meaning of the associated data.
Ports commonly implement flow-control by handshaking. Data is only transferred when both the sender and receiver are happy to proceed.
A port generally has an idle state which it returns to between each transaction.
Sometimes the start of one transaction is immediately after the end of the previous, so the transition through the idle state is only nominal.
Sometimes the begining of one transaction is temporaly overlaid with the end of a previous, so the transition through idle state has no absolute time associated with it.