Asetis a collection of distinct objects (calledelementsormembers) such that for any object, it can be determined whether the object belongs to the collection. --Academic Press Dictionary of Science and Technology

The order of elements of a set does not matter.
A given element only occurs once in a set.
A * multiset* or * bag* is
similar to a set but allows
multiple occurrences of a given element.
A set may be finite, such as the set of letters of the alphabet, or infinite,
such as the set of integers.

A finite set can be described by enumerating its members. An infinite
set can be described by giving a test for membership (* i.e.*,
a predicate).

In mathematical notation, a set is written inside braces:
* { a, e, i, o, u, y } *