"AC" is your armor class.  It is a number from -10 to 10, roughly
defined as follows:
  AC  Roughly equivalent to
  10  Naked person
   0  Very Heavily armored (Plate mail, and "two shields") 
 -10  Armored battle tank

The score command displays your armor class as a number such as 78/10;
this means 78 on a scale from -100 to 100 (i.e., 7.8 on the scale from
-10 to 10.)

See also: SCORE
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