flat-footed in the SRD
Using the search feature on a .pdf of the SRD, I found the following relevant references to flat-footed:
Balance
Being Attacked while Balancing: You are considered flat-footed while balancing, since you can’t move to avoid a blow, and thus you lose your Dexterity bonus to AC (if any). If you have 5 or more ranks in Balance, you aren’t considered flat-footed while balancing.
COMBAT REFLEXES [GENERAL]
Benefit: You may make a number of additional attacks of opportunity equal to your Dexterity bonus. With this feat, you may also make attacks of opportunity while flat-footed.
Normal: A character without this feat can make only one attack of opportunity per round and can’t make attacks of opportunity while flat-footed.
HOW COMBAT WORKS
Combat is cyclical; everybody acts in turn in a regular cycle of rounds. Combat follows this sequence:
1. Each combatant starts out flat-footed. Once a combatant acts, he or she is no longer flat-footed.
Flat-Footed: At the start of a battle, before you have had a chance to act (specifically, before your first regular turn in the initiative order), you are flat-footed. You can’t use your Dexterity bonus to AC (if any) while flat-footed. Barbarians and rogues have the uncanny dodge extraordinary ability, which allows them to avoid losing their Dexterity bonus to AC due to being flat-footed.
A flat-footed character can’t make attacks of opportunity.
SURPRISE
When a combat starts, if you are not aware of your opponents and they are aware of you, you’re surprised.
[...]
Unaware Combatants: Combatants who are unaware at the start of battle don’t get to act in the surprise round. Unaware combatants are flat-footed because they have not acted yet, so they lose any Dexterity bonus to AC.
GLOSSARY
Flat-Footed: A character who has not yet acted during a combat is flat-footed, not yet reacting normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.
AIMING A SPELL
Some spells restrict you to willing targets only. Declaring yourself as a willing target is something that can be done at any time (even if you’re flat-footed or it isn’t your turn).
Foresight [the spell]
... You are never surprised or flat-footed. ...