25 (twenty-five) is the natural number following 24 and preceding 26.

Cardinal	twenty-five
Ordinal	twenty-fifth
Factorization
Roman numeral	XXV
Binary	11001
Hexadecimal	19

In mathematics

It is a square number, being 5² = 5 × 5, and it is also the sum of two squares, 3² + 4², often used in demonstrations of the Pythagorean theorem. It is the smallest square that's also a sum of two squares. 25 is a centered octagonal number and an automorphic number.

It is also a Cullen number. 25 is the smallest pseudoprime satisfying the congruence 7ⁿ = 7 mod n. It is also a Somer pseudoprime based on 6.

Within base 10 one can readily test for divisibility by 25 by seeing if the last two digits of the number match 25, 50, 75 or 00.

In science

• The atomic number of manganese

In astronomy,

Messier object M25, a magnitude 4.9 open cluster in the constellation Sagittarius

The New General Catalogue object NGC 25 , a magnitude 11.9 barred spiral galaxy in the constellation Phoenix

The Saros number of the solar eclipse series which began on -2051 April 19 and ended on -771 May 26. The duration of Saros series 25 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -2038 August 6 and ended on -469 March 5. The duration of Saros series 25 was 1568.6 years, and it contained 88 lunar eclipses.

In other fields

Twenty-five is:

• The number of years of marriage marked in a silver wedding anniversary.
• The minimum age of candidates for election to the United States House of Representatives. "Under 25" provides a common cut-off point for designating youth.
• The number of cents in a quarter, an American coin with George Washington's portrait on the obverse and an eagle on the reverse (or special reverses for the special state quarters since 1999), and a Canadian coin with Queen Elizabeth II on the obverse and a moose on the reverse (or special reverses lately).
• The designation of United States Interstate 25, a freeway that runs from New Mexico to Wyoming.
• The designation of the M25 London Orbital motorway.
• The (critical) number of Florida electoral votes for the 2000 U.S. presidential election.
• Historical years: 25 A.D., 25 B.C., or (for example) 1925