51 is the natural number following 50 and preceding 52.

Cardinal	fifty-one
Ordinal	51st (fifty-first)
Factorization
Roman numeral	LI
Binary	110011
Hexadecimal	33

In mathematics

Fifty-one is a pentagonal number as well as a centered pentagonal number (one of the few numbers to be both) and an 18-gonal number. It is also the 6th Motzkin number, telling the number of ways to draw non-intersecting chords between any six points on a circle's boundary, no matter where the points may be located on the boundary.

In astronomy

• Messier object M51, a magnitude 8.0 spiral galaxy in the constellation Canes Venatici, also known as the Whirlpool Galaxy
• The New General Catalogue object NGC 51 , a spiral galaxy in the constellation Andromeda
• The Saros number of the solar eclipse series which began on -1407 September 2 and ended on 108 February 29. The duration of Saros series 51 was 1514.5 years, and it contained 85 solar eclipses. The Saros number of the lunar eclipse series which began on -1105 June 13 and ended on 211 August 12. The duration of Saros series 51 was 1316.2 years, and it contained 74 lunar eclipses.

In other fields

Fifty-one is also:

• The atomic number of antimony
• the code for international direct dial phone calls to Peru.
• the model number of the P-51 Mustang WWII fighter aircraft
• part of the title of a collection of stories by Lord Dunsany, Fifty-One Tales.
• Area 51, a parcel of U.S. military-controlled land in southern Nevada, apparently containing a secret aircraft testing facility.
• The year AD 51, 51 BC, and 1951.