A character who has not yet acted during a combat due to his or her Initiative is Flat-Footed, meaning the character is unprepared and can't react as well as when an attack is expected.
A Flat-Footed character...
- loses his Dexterity bonus (if he has one) to AC and to Combat Maneuver Defense (CMD).
- cannot make Attacks of Opportunity (AoO), unless he has the Combat Reflexes feat or Uncanny Dodge class ability.
- can be subject to Sneak Attacks.
The game may regard someone as being Flat-Footed also in other situations than in the beginning of a combat. For example the same penalties may apply when a web is inflicting the grappled condition on someone.