namespace is an abstract container providing context for the items (names, or technical terms, or words) it holds and allows disambiguation of items having the same name.

From the term "citizen," referring to a citizen of the Internet, or someone who uses networked resources.

Source: Sweeney, Susan

A computer network is two or more computers connected together using a telecommunication system for the purpose of communicating and sharing resources.

Source: wikipedia.org

Null is a special value for a pointer (or other kind of object reference) used to signify that the pointer intentionally does not have a target. Such a pointer is called a null pointer. Many implementations use a binary 0 (all bits zero) as the null value, as most operating systems consider it an error to try to access such a low memory address. Some languages use other nomenclature for such a pointer, e.g., nil, undefined, void reference, etc.

Source: wikipedia.org