"Prince" in HBP

[angellslin]

No: HPFGUIDX 106396

When everyone is guessing which boy/man the "Half Blood Prince" is 
referring, my question is - how come a prince emerges? 

As far as we know, wizarding world is ruled by Ministry of Magic. 
There're no king, queen, prince or princess. So what kind of prince 
is this character? How should we understand the term "prince"? Should 
we take it literally as a boy born to a royal family or what exactly 
does it mean? 


Angel