Pion is pronounced "pahy-on." 
 
Pion is named after the subatomic particle also known to physics geeks as a "pi meson." They are the lightest mesons, and play an important role in explaining low-energy properties of the strong nuclear force.
 
For more information see:
 
http://dictionary.reference.com/browse/pion
http://en.wikipedia.org/wiki/Pion