From Halopedia, the Halo wiki
Inconceivable  is a medal in Halo: Reach, Halo 4, Halo: The Master Chief Collection and Halo 5: Guardians that is awarded for killing 35 enemies in a row without dying in multiplayer and Spartan Ops, or for killing 500 enemies in campaign or Firefight without dying. In The Master Chief Collection it is only awarded in multiplayer (which includes Spartan Ops), and not in campaign, for which the highest killing spree medal is Invincible.
In Halo: Reach the medal is represented by a target on a red 32-point star. In Halo 4, The Master Chief Collection and Halo 5: Guardians it is a 14-point star.
In Halo: Spartan Assault for Xbox 360 and Xbox One, the Inconceivable medal is awarded for killing 40 enemies in a row in co-op without being hit, and is represented by a blue shield with a Pod infector over two inverted chevrons.
The name of the medal may be a reference to Vizzini's catchphrase in The Princess Bride.