(1) The set of names in a naming system.
(2)A logical grouping of the names used within a program. Also called name scope.
(3) A library of classes in .NET.
(4) XML namespace:
In XML, a namespace is a collection of names, identified by a URI reference, that are used in XML documents as element types and attribute names. In order for XML documents to be able to use elements and attributes that have the same name but come from different sources, there must be a way to differentiate between the markup elements that come from the different sources.
(5) DNS namespace:
DNS is the name service provided by the Internet for TCP/IP . DNS is broken up into domains, a logical organization of computers that exist in a larger network. The exist at different levels and connect in a hierarchy that resembles the root structure of a tree. Each domain extends from the node above it, beginning at the top with the root-level domain. Under the root-level domain are the top-level domains, under those are the second-level domains, and on down into subdomains. DNS namespace identifies the structure of the domains that combine to form a complete domain name. For example, in the, "com" is the top-level domain, "secondary" identifies the secondary domain name (commonly a site hosted by an organization and/or business), and "sub" identifies a subdomain within the larger network. This entire DNS domain structure is called the DNS namespace. The name assigned to a domain or computer relates to its position in the namespace.