A debuff in battle that causes the target to fall asleep for 3-5 turns. The target may not attack, use abilities, use items (eg. Alarm Clock), or escape from battle. Any source of damage, even MP damage from an ability like Pinch or Absorb, will cause a sleeping target to wake up. This in turn adds the Focus status to the target.
A sleeping player with an Alarm Clock in their inventory will automatically wake up when their turn in battle comes around, consuming the Clock in the process. Along those lines, Focus is applied when the target wakes up for any reason, not just when they take damage.