Oh no! Not again - I thought this very silly subject had been done to death before.
DAVE W - Since nobody seems to have answered your question, I shall.
In the UK and english speaking world generally, a Stall Turn is a vertical climb continued until flying speed no longer can be maintained, full rudder is applied to swing the aircraft about its "Z" axis to then face vertically downwards. A Hammerhead involves going backwards, erect or inverted (positive or negative).
However, in the USA things are a little different. There the Stall Turn is known as a Hammerhead. Why this should be so I am unable to say but they refuse to acknowledge our terminology on the basis that a stall is a condition pertinent to an aerofoil section only under load and since there is no load applied to the wing in this vertical manoeuvre, it cannot stall and in consequence the term is inappropriate. Pedantic perhaps but nevertheless correct.
For this reason it is prudent to get things clear with trans-Atlantic types. I once was teaching basic aeros in a Victa Airtourer to an American who was insistent that I showed him how to perform a Hammerhead and was quite upset at my refusal since the aircraft was not cleared for same. Back on the ground it became clear that he wanted to do Stall Turns - not "Tailslides" which is the American version of the Hammerhead.
GQ.