There are many ways of classifying sets. One way is by the size of the set: its cardinality.
On this basis a set may be
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An empty set in math is called a null set.
The theory of numbers is based on the property of sets of different kinds of numbers.
The null set is a set which has no members. It is an empty set.
An element is anything that is put into a set. In math, this can be just about anything you can talk about in math: for example, different types of numbers; points; polygons; lines; planes; matrices; operations; functions; sets; and many other more.
It is a set that is well defined.