Kill setting

From Memory Alpha, the free Star Trek reference.

Kill is a term used for a phaser setting strong enough to pose significant risk of death to a humanoid even from a short duration blast.

The effect of the kill setting can range anywhere from potentially lethal, where contact with a humanoid target usually causes severe injury or death, to maximum setting, which can disintegrate matter and usually kills humanoids even when exposed to the beam for a short duration.

Starfleet ships' sensors are set to trigger an alarm when a phaser set to kill is fired aboard ship without authorization. (Star Trek VI: The Undiscovered Country)

In 2366, Commander William T. Riker of the USS Enterprise-D was forced to fire his phaser on the kill setting at the Acamarian servant girl Yuta to prevent her from killing the Lornak tribe leader Chorgan. (TNG: "The Vengeance Factor")

When an elderly Kes traveled back in time to 2371, Captain Kathryn Janeway needed to stop her from kidnapping the Kes from the "present" timeline and assisting the Vidiians in destroying USS Voyager. Her telepathic powers had become so powerful, Captain Janeway was forced to set her phaser to kill to stop her. (VOY: "Fury")