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 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 NGC 81 , a spiral galaxy in the constellation Andromeda
• The Saros number 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. Further, the number of 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 atomic number of thallium
• 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
  

  

  Department 81 of France (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
• Number of stanzas in the Tao te Ching (in the most common arrangements).
• Baby 81, Abhilasha Jeyarajah, a baby survivor of the tsunami, claimed by many different parents.
• The year AD 81, 81 BC, or 1981.