A stealth kill is when an enemy is killed without even realizing anything is wrong. Stealth kills are considered great moves, as it requires skill to pull them off.
Once common strategy is to use remote view to watch an enemy around a corner, or behind a door. The player will watch them for a while, and then moves in once their path is known.
Another popular strategy is to crouch to sneak up on enemies. It makes it much less likely to be noticed by the target or anyone else.
Melee stealth kills are the stealth kill that requires more stealth. This is where the player sneaks up right behind an enemy, and melees them. They will die upon contact with the attack.
Mind Drain stealth kills WON'T instantly kill the enemy. First, sneak up behind an enemy, then HOLD the Mind Drain button. Use Mind Drain stealth kills only when there is only one person in a room (it might attract the attention of others). You have fully drained the enemy's life if the enemy's head explodes. The only problem with this stealth kill is that the enemy you're draining life out of is he will quickly recognize it.(but don't worry, the enemy can't do anything while being drained)
Another way to preform a stealth kill is telekinesis. The player uses telekinesis to left something while the enemy isn't looking, and throws it at them. It must kill them on contact to be a stealth kill.