1

The number one (1), also called "unity," is the first positive integer and the smallest natural number. One is the unit (the building block) of positive integers and can be used to make up all other integers. Any number that is multiplied or divided by one is unchanged. One is the first odd number, the first triangular, square, pentagonal, hexagonal, tetrahedral, and cube number. One is the only perfect nth power for all positive integers and the only positive integer with exactly one positive divisor.

Although one used to be considered a prime number, it requires special treatment in many definitions and applications involving primes greater than or equal to 2 that it is usually placed into a class of its own. A prime number is an integer greater than one with only one and itself as positive divisors (factors).