A more thorough explanation:

Definition: A tax or toll levied at a port or at the gates of a city. It can also refer to a toll for passing over a bridge.

Example: In ancient Rome, a portorium was collected on all goods entering the city through its gates.

Explanation: The example illustrates how a portorium was a tax collected at the gates of a city. It was a way for the city to generate revenue from goods entering the city. Similarly, a toll for passing over a bridge is another example of a portorium. It is a fee collected for using a specific infrastructure, in this case, a bridge.