Namespace is a context for identifiers, a logical grouping of names used in a program. Within the same context and same scope, an identifier must uniquely identify an entity.
In an operating system, a directory is a namespace. Each file or subdirectory within a directory has a unique name; the same name can be used multiple times across subdirectories.
In HTML, CSS, and XML-based languages, a namespace is the explicitly declared or implied dialect to which an element (or attribute) belongs.