The tap is specifically the thing that turns to control the flow of water. Your definition could just be a pipe. The tap can be opened or closed and this element of control should be included in your definition. From Concise Oxord: tap noun 1 a device by which a flow of liquid or gas from a pipe or container can be controlled.

I don't think it is the proper definition. Water tap is a spout connected to a socket to provide water from the main water supply. The correct definition using the above is probably like this. Water tap : A device which water flows through.

At least in the United States, tap and faucet are interchangeable. The important feature of a tap or faucet is that it controls the flow of water to enable the user to turn the water on or off.