A Sudoku puzzle

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).