A namespace is an element in a model that contains a set of named elements that can be identified by name. 

A project consists of one or more namespaces (or packages). A namespace and a package are almost synonyms: the term “namespace” is used for implementation projects, the term “package” is used for design projects. 

A namespace (or a package) is like a box where you put diagrams and model elements. Contents of a namespace (package) can be displayed on a special type of the Class Diagram. 

Each project contains the default namespace (or package) just after its creation.