AC probably refers to armor class from D&D usage. It's functionally different in Exiled Kingdoms, and is simply called armor here.
Here's the wiki page with info on character stats. Just search up the specific terms for more info.
http://www.exiledkingdoms.com/wiki/index.php?title=Character_Sheet
Edit*
Just to make things clear, AC in D&D serves to determine if an attack successfully hits the target. Exact calculations vary depending on which edition of D&D. Damage is calculated afterwards in a separate roll, only if the attack landed on the target.
In Exiled Kingdoms, all non-elemental attacks will deal damage. The amount of damage dealt/received is dependent on the armor stat, which subtracts from damage between 20% to 100% of the armor stat (40% to 100% if using a shield). I have seen different forms of this and
they have shown it too. The damage is of of the elements to consider. Non-elemental damage can only be reduced down to 20% here, so no invincibility for high armored characters.
When people type AC as a character stat on these forums, they're probably referring to the armor stat. Just that typing it as AC might be a habit from their experience coming from other games.