From Halopedia, the Halo wiki
The Beat Down medal (Assassin in Halo 2, Halo 3, Halo 3: ODST, Halo: Spartan Assault, and Halo: Spartan Strike) is awarded for killing an enemy with a melee from behind. In Halo: Reach , Halo 4, and Halo 5: Guardians the medal is awarded for killing an enemy with a melee from behind that does not result in an assassination. This is achieved by tapping rather than holding down the melee button. In Halo: The Master Chief Collection the medal is awarded in all games and in all modes, in accordance with the requirements of each original game.
In Halo 2 through Halo: Reach the medal is represented by a ninja on a silver eight-pointed star. In Halo 4 it is a silver ninja on a blue circle on a grey four-pointed star. In Halo 5: Guardians it is represented by a white ninja on a brown cross. In Spartan Assault and Spartan Strike it is represented by a shield with a punching fist.