The set of all possible labels or names within a system or a method of partitioning the set into subsets. Partititioned name spaces are greatly benificial for avoiding NamespaceCollisions.

C++, Java and XML all have partititioned NameSpaces, with XMLs being the most flexible due to provision for aliases. C and SGML lack NameSpaces, which is not a problem until you want to get two independantly designed and written systems to work together.