5

Five is the natural number that comes after four and before six. In mathematics, it is a prime number—the only prime that ends in the digit 5—and the third prime after 2 and 3. It is also a Fermat prime; for example, F1 = 2^(2^1) + 1 = 5. It is the second pentagonal number (P2 = 5) and appears in the Fibonacci sequence as the fifth Fibonacci number.

In geometry, a regular pentagon has five equal sides and five equal interior angles of 108 degrees. Its diagonals form a pentagram, and the ratio of a diagonal to a side in a regular pentagon is the golden ratio.

In numeral systems, five is written as 5 in base-10, as 101 in binary, and as V in Roman numerals, among other representations.

In science and culture, five has wide symbolic use: there are five classical elements in many traditions; there are five senses; the pentagon and pentagram have long-standing symbolic associations; the five Olympic rings symbolize continents. In chemistry, boron has atomic number 5.

In music, the interval of a perfect fifth and the pentatonic scale reflect the prominence of five-note structures. The month of May is the fifth month of the Gregorian calendar. The number also appears in many contexts as a common grouping or benchmark.