The Bone Cracker (Beat Down in Halo 3, Pummel in Halo: Reach, Melee in Halo 4, and Melee Kill in Halo: The Master Chief Collection and Halo 5: Guardians) medal is awarded for killing an opponent with a melee attack that does not assassinate him (a melee attack from behind).
In Halo 2 through Halo: Reach it is represented by an inverted five-point silver star with a dark green fist. In Halo 4 and The Master Chief Collection the medal is represented by a silver fist in a blue circle on a grey four-point star. In Halo 3 after you kill an opponent with a melee attack, a message appears on your screen that says: "You beat down (player's name)." In Halo: Reach the message "You pummeled (player's name)" will appear. In Halo 4 the message says "You meleed (player's name)."
In Halo 3, ODST, and Reach it also awarded if an Energy Sword or Gravity Hammer is used to kill an opponent with the right trigger. In Halo 4 these weapons award their own medals. In Halo 5: Guardians the Melee Kill medal is awarded for kills with the Gravity Hammer but not the Energy Sword, which again has its own medal.