Many pumps, when turned off, will still pass water. If there's any pressure at all upstream of the pump, the impeller will turn and water will leak through in the forward direction.
To prevent backflow of course, a check valve is used.
What's the term for a pump that forms a pressure seal when off, preventing flow in the pumped direction?
Best Answer
I believe the term you are looking for is a positive displacement pump.
However, such a pump is not generally constructed using an impeller and such a pump is only sometimes used for liquids.
Here is a link to a brief discussion of the characteristics of centrifugal pumps versus positive displacement pumps:
http://www.pumpschool.com/intro/pd%20vs%20centrif.pdf
For low pressure water applications a centrifugal pump and check valve in series are usually used to prevent back flow.