Takedown

A take down is a melee attack used to quickly and (In most cases) quietly dispose of an enemy. A take down can be lethal or non-lethal, with lethal take downs being slightly louder and more likely to alert others in the area. Any take down awards the “man down” (10 XP) and “expedient” (20 XP) bonuses and non-lethal take downs also earn the "Merciful Soul" (20XP) bonus, making a non-lethal take down worth 50 XP.

With the Quicksilver Reflex Booster augmentation, Adam Jensen can take down two targets at once if they are close enough, this yields the normal bonuses for the two take downs (60 XP for lethal 100 XP for non-lethal) along with a “two against on one” (25 XP) bonus (exactly, it gives only  one expedient (20 XP instead of 40 XP), but compensate for this by giving 45 XP for two against one; thus, saying 25 XP bonus is a simpler way to say the same thing). One take down requires one energy cell, whether the player performs it on one or two NPCs. This upgrade is highly recommended for stealth-oriented or pacifist characters.

Performing 50 take downs (on hostile characters) will award the player with the Opportunist Achievement. 100 nonlethal takedowns yields the Consciousness is Over-rated achievement; you will likely acquire both achievements when attempting a Pacifist playthrough.

Attempting to perform a takedown on a boss will cause the boss to counterattack; these counterattacks take off a significant chunk of Adam's health, and should be avoided.