For the year A.D. 51, see 51.
Fifty-one (51, LI) is the natural number following fifty and preceeding fifty-two. Its divisors are 1, 3, 17, and 51; its factorisation is
- The atomic number of antimony.
- The code for international direct dial phone calls to Peru.
- Part of the title of a collection of stories by Lord Dunsany, Fifty-One Tales.