Modularity
From Gephi:Wiki
Modularity

Measures how well a network decomposes into modular communities.
Objective
A high modularity score indicates sophisticated internal structure.
This structure, often called a community structure, describes how the the network is compartmentalized into subnetworks. These subnetworks (or communities) have been shown to have significant realworld meaning.
Explanation
Randomizing the algorithm can produce a better decomposition resulting in a higher modularity score, however randomizing will increase computation time.
Source code
See org.gephi.statistics.plugin.Modularity.java
.
Acknowledgments
This code was implemented by Patrick McSweeney.
Implemented Algorithm
Vincent D. Blondel, JeanLoup Guillaume, Renaud Lambiotte, Etienne Lefebvre  Fast unfolding of communities in large networks (2008) PDF