Usually the short form of the feminine "Stephanie" but in the cases of users of the spelling "Stephen" as the blunted spelling pronunciation "STeF-ən". As a bearer of the spelling I had been required to know this as being a part of my real first name despite the actual Canon as originally issued.
This is my name and I just love it. Well, my real name is Stephanie but lots of people call me Steph and it is weird when they call me Stephanie. :P Coz it is like my real name is Steph and they call me Stephanie. But if you have a kid, call it Steph or Stephanie.