Some collection of classes or library functions grouped as one name space. A class which belongs to one namespace is different from the class which belongs to another namespace. we can identify a class uniquely with it's namespace .
for ex:
in c#.net
using system;
System.IO;
here System is the namespace which contains class IO
namespace contains any number of classes . In one namespace we can't define two classes with same Name. We can define two classes with same name in different namespaces
