Awareness

Awareness is the state of being aware of another creature. Unlike Alertness, Awareness is specific to a target: you can be Aware of one creature but not of another.

You are automatically Aware of all creatures you can see.

If you cannot see a creature clearly, but you have other sensory information (blurred vision, hearing, smell, etc) which can pinpoint the creature's location to a single square (or squares, if large), then you are Partially Aware of the creature. That means you are not Unaware, but you suffer -10 to active defenses against that target, and any attacks against the target are made as if Blind (i.e. 50% miss chance, unable to purposefully critically hit).

If you are not Aware or Partially Aware of a creature, you are Unaware of it, and cannot use active defenses against its attacks.