84 (eighty-four) is the natural number following 83 and preceding 85.

In mathematics

84 is a semiperfect number,^[1] being thrice a perfect number, and the sum of the sixth pair of twin primes ${\displaystyle (41+43)}$.^[2]

It is the third (or second) dodecahedral number,^[3] and the sum of the first seven triangular numbers (1, 3, 6, 10, 15, 21, 28, 36), which makes it the sixth tetrahedral number.^[4]

The twenty-second unique prime in decimal, with notably different digits than its preceding (and known following) terms in the same sequence, contains a total of 84 digits.^[5]

A hepteract is a seven-dimensional hypercube with 84 penteract 5-faces.

84 is the limit superior of the largest finite subgroup of the mapping class group of a genus ${\displaystyle g}$ surface divided by ${\displaystyle g}$.^{[citation needed]}

Under Hurwitz's automorphisms theorem, a smooth connected Riemann surface ${\displaystyle X}$ of genus ${\displaystyle g>1}$ will contain an automorphism group ${\displaystyle \mathrm {Aut} (X)=G}$ whose order is classically bound to ${\displaystyle |G|\leq 84{\text{ ))(g-1)}$.^[6]

There are 84 zero divisors in the 16-dimensional sedenions ${\displaystyle \mathbb {S} }$.^[7]

In astronomy

• Messier object M84, a magnitude 11.0 lenticular galaxy in the constellation Virgo
• The New General Catalogue object NGC 84, a single star in the constellation Andromeda

In other fields

Eighty-four is also:

• The year AD 84, 84 BC, or 1984.
• The number of years in the Insular latercus, a cycle used in the past by Celtic peoples,^[8] equal to 3 cycles of the Julian Calendar and to 4 Metonic cycles and 1 octaeteris
• The atomic number of polonium
• The model number of Harpoon missile
• WGS 84 - The latest revision of the World Geodetic System, a fixed global reference frame for the Earth.
• The house number of 84 Avenue Foch
• The number of the French department Vaucluse
• The code for international direct dial phone calls to Vietnam
• The town of Eighty Four, Pennsylvania
• The company 84 Lumber
• The ISBN Group Identifier for books published in Spain
• A variation of the game 42 played with two sets of dominoes.
• The film 84 Charing Cross Road (1987) starring Anne Bancroft and Anthony Hopkins
• KKNX Radio 84 in Eugene, Oregon
• The B-Side to "Up All Night" (Take That song)
• British Army term for the 84mm Carl Gustav recoilless rifle.
• How many Earth years it takes Uranus to orbit the Sun once
• The total number of Vertcoin to be released is 84 million “Vertcoin”
• The number of former National Football League player Antonio Brown