A51 may refer to:
*
Area 51, the nickname for a military base in Nevada that is the subject of many conspiracy theories
*
A51 Terrain Park (Colorado), a terrain park in Keystone, Colorado
*
A51 road (England), a road connecting Kingsbury and Chester
*
A51 motorway (France), a road connecting Marseille and Grenoble
*
A5/1
A5/1 is a stream cipher used to provide over-the-air communication privacy in the GSM cellular telephone standard. It is one of several implementations of the A5 security protocol. It was initially kept secret, but became public knowledge through l ...
, in cryptography, a stream cipher used in GSM cellular networks
*
Samsung Galaxy A51, a smartphone released in 2019
* A51, one of the Encyclopaedia of Chess Openings codes for the
Budapest Gambit in chess
* A-51, a
Namibian hip hop band
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