For namespaces with respect to Wikipedia, see Wikipedia:Namespace.
In general, a namespace is an abstract container, which is or could be filled by names, or technical terms, or words, and these represent (stand for) real-world things.
As a rule, names in a namespace cannot have more than one meaning, that is, two or more things cannot share the same name. A namespace is also called a context, as the valid meaning of a name can change depending on what namespace applies. Names in it can represent objects as well as concepts.
Each language is a namespace, whether it is a natural or ethnic language, a constructed language, the technical terminology of a profession, a dialect, a sociolect, or an artificial language (e.g. a programming language).
For many programming languages, a namespace is a context for identifiers.
Illustration
Within a limited namespace designed for your family, you might be named as "Charlie". Within a larger namespace containing strangers as well, the name "Charlie" might not be unique, so you are instead "Charlie Brown, 17 Main Street". In (the namespace of) some other family, the name "Charlie" might refer to a different person than you.
Examples
See also