21 (twenty-one) is the natural number following 20 and preceding 22.

Cardinal	twenty-one
Ordinal	twenty-first
Factorization
Roman numeral	XXI
Binary	10101
Hexadecimal	15

In mathematics

Twenty-one is a Fibonacci number, a Harshad number, a Motzkin number, a triangular number and an octagonal number, as well as a composite number, its proper divisors being 1, 3 and 7.

In science

• The atomic number of scandium

In astronomy,

Messier object M21, a magnitude 7.0 open cluster in the constellation Sagittarius

The New General Catalogue object NGC 21 , a barred spiral galaxy in the constellation Andromeda

The Saros number of the solar eclipse series which began on -2275 May 5 and ended on -995 June 11. The duration of Saros series 21 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number of the lunar eclipse series which began on -1973 February 11 and ended on -639 April 22. The duration of Saros series 21 was 1334.2 years, and it contained 75 lunar eclipses.

In human affairs: age 21

21 had been the age of majority in England and Wales before that age was lowered to 18 with the coming into force, on 1 January 1970, of the Family Law Reform Act 1969.

In the United States, when one reaches 21, one gains the right to drink alcohol. It is also the age at which it is legal to gamble and have sex with a prostitute in Nevada. In most states, a person who is 21 or over may also rent a car. 21 was the age for voting in the majority of states until the introduction of the Twenty-sixth Amendment.

In other fields

The current century -- spanning the years from 2001 to 2100 -- is referred to as the 21st century. This inspired many forward-looking 20th century event, company, and product names, like the Century 21 Exposition (another name for the 1962 Seattle World's Fair), Century 21 Real Estate, and Century 21 Television (producers of Sylvia and Gerry Anderson Supermarionation shows like Thunderbirds and Captain Scarlet).

The Twenty-first Amendment repealed the Eighteenth Amendment, thereby ending Prohibition.

Pier21 was, from 1928 to 1971, the place where immigrants entered Canada. It was called the "Gateway to Canada".

Twenty-one is:

• The number of spots on a standard cubical die (1+2+3+4+5+6).
• The number of firings in a gun salute to Royalty or leaders of countries.
• A card game, also called vingt-et-un (French for "twenty-one"), pontoon, or blackjack.
• The title of a TV quiz show that ran from 1956 to 1958, most remembered for the scandal that the wins were fixed (see Quiz show scandals). It was remade in 2000.
• The title of at least three different movies:
  • A 1918 comedy starring Bryant Washburn and Gertrude Selby
  • A 1923 drama starring Richard Barthelmess and Dorothy Mackaill
  • A 1991 drama starring Patsy Kensit
• in the title of the tv show 21 Jump Street, starring Johnny Depp
• the standard TCP/IP port number for FTP connection.
• Twenty-One Demands were a set of demands which were sent to the Chinese government by the Japanese government of Okuma Shigenobu in 1915.
• Number 21, the plane flown by Gustave Whitehead
• Historical years: 21 A.D., 21 B.C., or 1921
• The title of a song by the Cranberries
• 21 is the title of a novel by Patrick O'Brian
• A street variation on basketball in which each player, of which there can be any number, plays for himself only (i.e. not part of a team); the name comes from the requisite number of baskets.
• The "21 Club" is a top restaurant in New York City.
• In Israel, the number is associated with the profile 21 (the military profile designation granting an exemption from the military service).