81 is the natural number following 80 and preceding 82.

In mathematics

Eighty-one is a square, a heptagonal number and a centered octagonal number. It is also a tribonacci number, an open meandric number and a Harshad number.

In science

• The atomic number of thallium

In astronomy,

Messier object M81, a magnitude 8.5 spiral galaxy in the constellation Ursa Major, also known as Bode's Galaxy, and the first of what is known as the M81 Group of galaxies

The New General Catalogue object (http://www.ngcic.org/) NGC 81, a spiral galaxy in the constellation Andromeda

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/SEsaros/SEsaros1-175.html) of the solar eclipse series which began on -322 May 12 and ended on 958 June 19. The duration of Saros series 81 was 1280.1 years, and it contained 72 solar eclipses.

The Saros number (http://sunearth.gsfc.nasa.gov/eclipse/LEsaros/LEsaros1-175.html) of the lunar eclipse series which began on -20 February 19 and ended on 1296 April 19. The duration of Saros series 81 was 1316.2 years, and it contained 74 lunar eclipses.

In other fields

Eighty-one is also:

• the title of a short film by Stephen Burke: 81
• the model number of: Unterseeboot 81, Sinclair ZX81
• the number of the French department Tarn
• the designation of Interstate 81, a freeway that runs from Tennessee to New York.
• the code for international direct dial phone calls to Japan
• the number of times a popular rhyme (erroneously) claims Lizzie Borden struck her parents with an axe
• One of two ISBN Group Identifiers for books published in India
• The year AD 81, 81 BC, or 1981.