What term describes when a character (pc or npc) is officially in combat and no longer flat-footed?
Can a character ready an action to attack a target for when they are no longer flat-footed, in order to have the attack happen before they loose the flat-footed condition?
No, Flat-Footed is not an action thus it is not really possible to ready against it. However, some GMs may state it is an observable condition (maybe with a Sense Motive check). Personally, I wouldn't allow it.