what is a physical interface. definition
 

The networking dictionary i have has no definition for this term: physical interface.
This is verbatim from a textbook:
What does the author mean by "physical interface"? does he mean a gateway like a router or bridge, or a modem? thanks

From what i understand, a node is a workstation and a host is a gateway computer, so what will a physical interface be compared to?
The topology referred therein is a mesh topology.

No, it means something like an ethernet card inside a PC.
If the mesh topology represents a fully connected network, every host must have a direct connection to all the other hosts, so that's why every host needs n-1 physical interfaces (the nth one would be needed to connect to itself, which is unneded.

See this for a picture of full mesh. http://en.wikipedia.org/wiki/Network_topology

Understanding what 'mesh' is would be an overkill without understanding what a physical interface is.

Basically, a physical media like a fastethernet or a optic fiber, connect nodes (computers, servers, etc). At both ends of such a media, there's an "interface" such as a LAN card, as mentioned above. This physical interface allows the software in the device to be "connected" to the physical medium, i.e. wire/fastethernet/optic fiber.