1
1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of unit length is a line segment of length 1. In conventions of sign where zero is considered neither positive nor negative, 1 is the first and smallest positive integer.[1] It is also sometimes considered the first of the infinite sequence of natural numbers, followed by 2, although by other definitions 1 is the second natural number, following 0.
The fundamental mathematical property of 1 is to be a multiplicative identity, meaning that any number multiplied by 1 equals the same number. Most if not all properties of 1 can be deduced from this. In advanced mathematics, a multiplicative identity is often denoted 1, even if it is not a number. 1 is by convention not considered a prime number; this was not universally accepted until the mid-20th century. Additionally, 1 is the smallest possible difference between two distinct natural numbers.
The unique mathematical properties of the number have led to its unique uses in other fields, ranging from science to sports. It commonly denotes the first, leading, or top thing in a group.
One is most commonly a determiner used with singular countable nouns, as in one day at a time.[2] One is also a pronoun used to refer to an unspecified person or to people in general as in one should take care of oneself.[3] Finally, one is a noun when it refers to the number one as in one plus one is two and when it is used as a pro form, as in the green one is nice or those ones look good.
One comes from the English word an,[4] which comes from the Proto-Germanic root *ainaz.[4] The Proto-Germanic root *ainaz comes from the Proto-Indo-European root *oi-no-.[4]
Compare the Proto-Germanic root *ainaz to Old Frisian an, Gothic ains, Danish en, Dutch een, German eins and Old Norse einn.
