ac

"AC" is your armor class.  It is a number from -100 to 100, roughly
defined as:

armored like a tank        -100
extremly shielded          -90
heavily shielded           -80
shielded                   -70
extremly heavily armored   -60
extremly armored           -50
very heavily armored       -40
heavily armored            -30
very good armored          -20
good armored               -10
completely armored           0 
fully armored               10
armored                     20
slightly armored            30
fully protected             40
protected                   50
slightly protected          60
fully clothed               70
clothed                     80
almost naked                90
naked                       100

A piece of armor with AC 8 will set your armor from 100 to 92, for example.
The lower your AC, the more will the damage of the enemies stroke be reduced.
Sometimes items gives boni or mali to 'ARMOR'. A negative ARMOR improves
your AC. for example, a sword giving -3 to Armor will decrease your AC
from 100 to 97.
Note: Items with AC (not armor) worn on head, legs and body protect more than
      other pieces of armor.