The Law of Demeter (LoD), or principle of least knowledge, is a design principle related to loose coupling. In order to minimize coupling between software modules, the principle can be followed when designing software.
The principle follows the only talk to your friends idiom, which keeps coupling loose by limiting a module's communication with other modules. Ideally, a method should only call other methods in the same object, in objects that were passed into it, in direct component objects, in objects that it created/instantiated, or in objects in a global variable that are accessible.
Another one of the tenets of LoD is that a software module should know as little as possible about other modules. This will ensure its independence from other modules, allowing coupling to remain loose. See the Information hiding section for a principle that helps to achieve this goal.