Understanding the Meaning of 'p' in Equation: p^2 = (4pi^2)(a^3)/G(m1+m2)

[aerogurl2]

hi, there's something that i don't understand in the following equation:

p^2 = (4pi^2)(a^3)/G(m1+m2)

is the p a distance measured from the center of the first object to the center of the second object? or is it just the distance between them, for example a satelitte dish is 300km above the Earth's surface, so would p be 300km or 300km+radius of earth?

 

[andrevdh]

http://www.allrefer.com/kepler-s-laws-of-planetary-motion"