Why the "½" in the title of "Ranma ½"?
It seems is just to describe he is a man and a woman.  Is because of the gender switch  


  Rumiko Takahashi stated that Ranma ½ was conceived to be a martial arts manga that connects all aspects of everyday life to martial arts. Because her previous series had female protagonists, the author decided that she wanted a male this time. However, she was worried about writing a male main character, and therefore decided to make him half-female. Before deciding on water for initiating his changes, she considered Ranma changing every time he was punched. It was after deciding this that she felt Jusenkyo had to be set in China, as it is the only place that could have such mysterious springs


Source: Ranma ½ > Production