For the full description, implementation variations, consequences, and detailed sample, see Page 116 in Java Application Architecture.
Design module relationships.
A relationship between two modules exists when a class within one module imports at least a single class within another module. In other words:
If changing the contents of a module, M2, may impact the contents of another module, M1, we can say that M1 has a Physical Dependency on M2. [JOUP02]