1) Empty math class 2) Group with no members 3) It has no members 4) Mathematical void 
In mathematics, a null set is a set that is negligible in some sense. For different applications, the meaning of `negligible` varies. In measure theory, any set of measure 0 is called a null set (or simply a measure-zero set). More generally, whenever an ideal is taken as understood, then a null set is any element of that ideal. The remainder of.....
(from the article `formal logic`) ...is known as the principle of extensionality. A class with no members, such as the class of Chinese popes, is said to be null. Since the membership ... ...der Arithmetik (1884; The Foundations of Arithmetic). He regarded two sets as the same if they contained the same elements. So in his opinion ... ...
[n] - a set that is empty
The null set is a set that contains no objects. Also known as the empty set. 
noun a set that is empty; a set with no members