Modularity "Modularity is a general systems concept, typically defined as a continuum describing the degree to which a system's components may be separated and recombined. It refers to both the tightness of coupling between components, and the degree to which the "rules" of the system architecture enable (or prohibit) the mixing and matching of components."

From - http://en.wikipedia.org/wiki/Modularity

The opposite of modularity is monolithic design. Try to compare and contrast how these two design principles can serve and/or dis-serve application, operating system, and computer security. https://en.wikipedia.org/wiki/Monolithic_kernel