In mathematics, an element (also called a member) is an object contained in a set (or more generally a class).
Writing "A = {1, 2, 3, 4}", means that the elements of the set A are the numbers 1, 2, 3 and 4. Groups of elements of A, for example {1, 2}, are subsets of A.
Elements can themselves be sets. For example consider the set B = {1, 2, {3, 4} }. The elements of B are not 1, 2, 3, and 4. Rather, there are only three elements of B, namely the numbers 1 and 2, and the set {3, 4}.
The elements of a set can be anything. For example, C = {red, green, blue}, is the set whose elements are the colors red, green and blue.
The relation "is an element of", also called set membership, is denoted by "∈", and writing "x ∈ A", means that x is an element of A. Equivalently one can say or write "x is a member of A", "x belongs to A", "x is in A", or A contains x. The negation of set membership, is denoted by "∉".
Examples (using the sets defined above):
