2

The number 2 is the second natural number, after 1. Each natural number, including 2, is constructed by succession, that is, by adding 1 to the previous natural number.^[1] 2 is the smallest and the only even prime number, and the first Ramanujan prime.^[2] It is also the first superior highly composite number,^[3] and the first colossally abundant number.^[4]

An integer is determined to be even if it is divisible by two. When written in base 10, all multiples of 2 will end in 0, 2, 4, 6, or 8;^[5] more generally, in any even base, even numbers will end with an even digit.

Binary is a number system with a base of two, where each "bit" (binary digit) is either 0 (off) or 1 (on). It is used extensively in computing, since simple on-off logic is relatively simple to keep track of with electronics.^[6]

A digon is a polygon with two sides (or edges) and two vertices.^[7]: 52 In Euclidean space, digons are degenerate, collapsing to a line segment between the two vertices.^[8] In spherical geometry, however, non-degenerate digons can exist.^[9]

Two distinct points in a plane are always sufficient to define a unique line in a nontrivial Euclidean space.^[10]

Two is most commonly a determiner used with plural countable nouns, as in two days or I'll take these two.^[11] Two is a noun when it refers to the number two as in two plus two is four.

The word two is derived from the Old English words twā (feminine), tū (neuter), and twēġen (masculine, which survives today in the form twain).^[12]

The digit used in the modern Western world to represent the number 2 traces its roots back to the Indic Brahmic script, where "2" was written as two horizontal lines. The modern Chinese and Japanese languages (and Korean Hanja) still use this method. The Gupta script rotated the two lines 45 degrees, making them diagonal. The top line was sometimes also shortened and had its bottom end curve towards the center of the bottom line. In the Nagari script, the top line was written more like a curve connecting to the bottom line. In the Arabic Ghubar writing, the bottom line was completely vertical, and the digit looked like a dotless closing question mark. Restoring the bottom line to its original horizontal position, but keeping the top line as a curve that connects to the bottom line leads to our modern digit.^[13]

• The first magic number - number of electrons in the innermost electron shell of an atom.^[14]

• Binary number
• Square root of 2
• −2