Frightened talks only about DCs:
[...] You take a status penalty equal to this value to all your checks and DCs [...]
Compare it to Clumsy:
[...] You take a status penalty equal to the condition value to Dexterity-based checks and DCs, including AC, Reflex saves, ranged attack rolls, and skill checks [...]
The wording of Clumsy seems to imply that AC is just one of the DCs, but is not clear. I really hope it is, otherwise Frightened is not nearly as good as I thought.
Armor Class is referenced in multiple locations as a DC. Most notably, in the rules in Equipment about AC:
Your Armor Class (AC) measures how well you can defend against attacks. When a creature attacks you, your Armor Class is the DC for that attack roll.
And also Playing the Game Key Terms
All creatures in the game have an Armor Class. This score represents how hard it is to hit and damage a creature. It serves as the Difficulty Class for hitting a creature with an attack.
Because this is a little ambiguous (being "the DC for that ... roll" vs "you have a DC that is your AC"), you can also reference the rules on attacks:
Attack rolls are compared to a special difficulty class called an Armor Class (AC), which measures how hard it is for your foes to hit you...
