A number is a concept from mathematics, used to count or measure. Depending on the field of mathematics, where numbers are used, there are different definitions:

• People use symbols to represent numbers; they call them numerals. Common places where numerals are used are for labeling, as in telephone numbers, for ordering, as in serial numbers, or to put a unique identifier, as in an ISBN, a unique number that can identify a book.
• Cardinal numbers are used to measure how many items are in a set. For example, {A,B,C} has size "3".
• Ordinal numbers are used to specify a certain element in a set or sequence (first, second, third).

Numbers are also used for other things like counting. Numbers are used when things are measured. Numbers are used to study how the world works. Mathematics is a way to use numbers to learn about the world and make things. The study of the rules of the natural world is called science. The work that uses numbers to make things is called engineering.