25 unaware enemies killed.
An unaware enemy is obvious. They aren't attacking you and are in a calm state. You can kill them with a head shot or sneak up behind them and silent kill them using the Y prompt. If there are multiple enemies around, you will need to use the bow. If you look in the DLC area of the game, you may also purchase (using Microsoft Points) a silenced pistol. This is not needed, however.

Note: You cannot reload checkpoints to gain extra kills towards your stats. These must all be done on unique enemies.